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      10-01-2021, 12:53 AM   #1
delta
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Question: "Clunk and shudder" at shut-down: The M Rear Axle Differential Lock?

Picked up a '21 LCI. First M5. I now have just over 2K miles. Enjoying the heck out of it.

But, as I've been getting used to the sounds and operating characteristics that any new ride presents, I've been noticing that when I shut off the ignition, I often get a one-time, audible "clunk" sound coming from the rear of the car. Not a loud nor metallic sound, but one that is definitely audible. Just a dull, summary clunk that I distinctly hear coming from the rear trunk/below trunk area just after pushing the engine start button to shut-off.

Perhaps as important a clue, the sound is accompanied by an abrupt shudder that I can feel through the driver's seat.

The noise and accompanying shudder does not occur every time. When I shut off the car on a level surface, it rarely occurs; but, I've noticed that if I shut off on an incline, or just after driving up an inclined surface to a level surface, and then shut off the ignition, I almost always, if not always, hear that "clunk" and hopefully benign, accompanying shudder.

--Is the "clunk" and accompanying shudder related to some kind of unloading of the M5's M rear axle differential lock?

--Is it some type of lock-up tension suddenly released from the differential at the time of ignition shut-off?

--Possibly an issue with the diffy's lock control unit?

I am not concerned at this point; Perhaps I should...

TIA!
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      10-01-2021, 08:51 AM   #2
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Just a thought, to eliminate a possibility.
While holding the brake down, put the emergency brake on, then put into park, then release the normal foot brake.
Just wondering if its the tranny holding the weight when on a incline.
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      10-01-2021, 12:38 PM   #3
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Quote:
Originally Posted by Danny D View Post
Just a thought, to eliminate a possibility.
While holding the brake down, put the emergency brake on, then put into park, then release the normal foot brake.
Just wondering if its the tranny holding the weight when on a incline.
I did try doing which you suggest. As with other auto trans cars I've owned, the M5 had almost an imperceptible (normal) sag upon releasing the foot brake after putting on the emergency brake and the car into Park on my driveway incline (slight slope). Nothing remarkable there, and definitely not the shudder that I described in my first post.

Thanks for the thought though, Danny.
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      10-02-2021, 10:20 AM   #4
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Hey folks,

Any other comments here on my shut-off shudder issue? Normal behavior? A problem brewing? I'm new to this M5 community, but not to M.

That said, this one has me stumped. My gut sez this is a normal operating characteristic, but would like confirmation.

Opinions would be appreciated. (And thanks again, Danny.)

...terry
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      10-05-2021, 10:36 PM   #5
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Quote:
Originally Posted by delta View Post
Picked up a '21 LCI. First M5. I now have just over 2K miles. Enjoying the heck out of it.

But, as I've been getting used to the sounds and operating characteristics that any new ride presents, I've been noticing that when I shut off the ignition, I often get a one-time, audible "clunk" sound coming from the rear of the car. Not a loud nor metallic sound, but one that is definitely audible. Just a dull, summary clunk that I distinctly hear coming from the rear trunk/below trunk area just after pushing the engine start button to shut-off.

Perhaps as important a clue, the sound is accompanied by an abrupt shudder that I can feel through the driver's seat.

The noise and accompanying shudder does not occur every time. When I shut off the car on a level surface, it rarely occurs; but, I've noticed that if I shut off on an incline, or just after driving up an inclined surface to a level surface, and then shut off the ignition, I almost always, if not always, hear that "clunk" and hopefully benign, accompanying shudder.

--Is the "clunk" and accompanying shudder related to some kind of unloading of the M5's M rear axle differential lock?

--Is it some type of lock-up tension suddenly released from the differential at the time of ignition shut-off?

--Possibly an issue with the diffy's lock control unit?

I am not concerned at this point; Perhaps I should...

TIA!
Can't say I've noticed this (2018 model here) and I'm pretty sensitive to these types of things, but I'll keep my ears piqued.
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      10-07-2021, 11:54 AM   #6
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Quote:
Originally Posted by Demon Barber View Post
Can't say I've noticed this (2018 model here) and I'm pretty sensitive to these types of things, but I'll keep my ears piqued.
Appreciate the watchful input. Having awareness in this regard (i.e., noticing when something is amiss car-wise) is certainly a plus.
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      10-07-2021, 11:43 PM   #7
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Quote:
Originally Posted by delta View Post
Picked up a '21 LCI. First M5. I now have just over 2K miles. Enjoying the heck out of it.

But, as I've been getting used to the sounds and operating characteristics that any new ride presents, I've been noticing that when I shut off the ignition, I often get a one-time, audible "clunk" sound coming from the rear of the car. Not a loud nor metallic sound, but one that is definitely audible. Just a dull, summary clunk that I distinctly hear coming from the rear trunk/below trunk area just after pushing the engine start button to shut-off.

Perhaps as important a clue, the sound is accompanied by an abrupt shudder that I can feel through the driver's seat.

The noise and accompanying shudder does not occur every time. When I shut off the car on a level surface, it rarely occurs; but, I've noticed that if I shut off on an incline, or just after driving up an inclined surface to a level surface, and then shut off the ignition, I almost always, if not always, hear that "clunk" and hopefully benign, accompanying shudder.

--Is the "clunk" and accompanying shudder related to some kind of unloading of the M5's M rear axle differential lock?

--Is it some type of lock-up tension suddenly released from the differential at the time of ignition shut-off?

--Possibly an issue with the diffy's lock control unit?

I am not concerned at this point; Perhaps I should...

TIA!
Quote:
Originally Posted by delta View Post
Picked up a '21 LCI. First M5. I now have just over 2K miles. Enjoying the heck out of it.

But, as I've been getting used to the sounds and operating characteristics that any new ride presents, I've been noticing that when I shut off the ignition, I often get a one-time, audible "clunk" sound coming from the rear of the car. Not a loud nor metallic sound, but one that is definitely audible. Just a dull, summary clunk that I distinctly hear coming from the rear trunk/below trunk area just after pushing the engine start button to shut-off.

Perhaps as important a clue, the sound is accompanied by an abrupt shudder that I can feel through the driver's seat.

The noise and accompanying shudder does not occur every time. When I shut off the car on a level surface, it rarely occurs; but, I've noticed that if I shut off on an incline, or just after driving up an inclined surface to a level surface, and then shut off the ignition, I almost always, if not always, hear that "clunk" and hopefully benign, accompanying shudder.

--Is the "clunk" and accompanying shudder related to some kind of unloading of the M5's M rear axle differential lock?

--Is it some type of lock-up tension suddenly released from the differential at the time of ignition shut-off?

--Possibly an issue with the diffy's lock control unit?

I am not concerned at this point; Perhaps I should...

TIA!
In all honesty it could just be the car shutting down. The new M cars and frankly all new BMWs shutter a lot. Even my 2021 750i does in sport mode.

For our M5s, mine also being a 2021, If you put the car in it's sportiest settings and tap the throttle just a tiny bit, the entire car shutters.
The same effect happens when the fuel is cut off when you shut it down.
This was also relevant in my 2019 comp.

I know it seems weird but it really is normal.
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      10-08-2021, 01:37 AM   #8
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Quote:
Originally Posted by Jacob_Tyler_Freed View Post
In all honesty it could just be the car shutting down. The new M cars and frankly all new BMWs shutter a lot. Even my 2021 750i does in sport mode.

For our M5s, mine also being a 2021, If you put the car in it's sportiest settings and tap the throttle just a tiny bit, the entire car shutters.
The same effect happens when the fuel is cut off when you shut it down.
This was also relevant in my 2019 comp.

I know it seems weird but it really is normal.
Understand. Candidly, my '13 GTR's dual-clutch/rear transaxle/AWD differential made a surprising mix of sounds that were, as I later learned, quite normal!

So, I gotta go once again with those who know. Appreciate your thoughts.
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      10-09-2021, 10:54 AM   #9
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What is your shutdown sequence? Is your foot on the brake pedal? Do you use the auto-hold feature all the time? Do you apply the parking brake before you shut down the car or let the car apply it on its own?

The rear parking brake with automechanical actuation is used for a lot of things, including auto-hold, short-term false “Park” when left idling for a long time until the transmission truly engages Park several minutes later.

All of these things can be and often are audible and felt.
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      10-10-2021, 07:02 AM   #10
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OP - this is exactly the same thing happening in my 21 LCI. This has nothing with the sequence of shutting down. There is definitely something going on in the rear and it is not consistently happening every time but more so than not.

I'll ask my service advisor during the next check in.

This never happened in my 2019 pre-LCI. I posted it earlier in the year after having the car for a month or so - see my third point:

https://f90.bimmerpost.com/forums/sh....php?t=1834085
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      10-10-2021, 01:11 PM   #11
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Quote:
Originally Posted by ashmostro View Post
What is your shutdown sequence? Is your foot on the brake pedal? Do you use the auto-hold feature all the time? Do you apply the parking brake before you shut down the car or let the car apply it on its own?

The rear parking brake with automechanical actuation is used for a lot of things, including auto-hold, short-term false “Park” when left idling for a long time until the transmission truly engages Park several minutes later.

All of these things can be and often are audible and felt.
Appreciate your reply.

I have tried numerous sequences in a quest to isolate the problem. I cut the ignition first with the brake pedal on and with the brake pedal off; I applied the parking brake before cutting the ignition, and I tried the various sequences with the auto-hold on and off before cutting the ignition. I concluded that ignition cut-off, with or without the presence of the foot brake, parking brake or auto-hold, was the initiator of the problem sound.
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      10-10-2021, 01:13 PM   #12
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Thanks.

In that case, the first thing I’d suggest is to ask your dealer to check the rear parking brake and potentially reset/adjust it. It might not be functioning within spec.

Good luck and let us know what turns up!
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      10-10-2021, 02:11 PM   #13
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Quote:
Originally Posted by wesleyan92 View Post
OP - this is exactly the same thing happening in my 21 LCI. This has nothing with the sequence of shutting down. There is definitely something going on in the rear and it is not consistently happening every time but more so than not.

I'll ask my service advisor during the next check in.

This never happened in my 2019 pre-LCI. I posted it earlier in the year after having the car for a month or so - see my third point:

https://f90.bimmerpost.com/forums/sh....php?t=1834085
Just read your earlier June 2021 thread entitled "My LCI - is it all in my head." Interesting to note the following comments on that June 2021 thread:

wesleyan92 remarked: "Also, when I turn the car off, 50% of the time, I feel a clunk/thud in my rear end coming from the bottom of the car. Super weird."

PunjabiM3 remarked: "I have had mine since Jan 2nd and have also been experiencing this thud coming from the rear when putting the car into park. I wonder if perhaps there is something wrong with the rear diff? What is the build date on your car? Please post if you figure out what it is coming from, I have thought about taking the car to have it looked at but it is periodic and nothing that sounds bad, at least not yet."

While three positive responses (including mine) doesn't a definite survey trend make, the issue is rather unusual and worthy of at least a raised eyebrow or two. Something is definitely happening in the M5's rear drivetrain/parking brake componentry just after ignition cut-off. What is it?

At this point, I'll be following your strategy for now, i.e., "...it's not that bad, is not getting worse, and doesn't impact any performance or operation." That said, though I just brought the car in for its 1200 mile break-in a few weeks back, I plan to check back in with the dealer to make this seemingly innocuous issue a matter of record. You know. Just in case...

My build date is April/2021, BTW.

Last edited by delta; 10-10-2021 at 02:22 PM.. Reason: Added the possibility of a maladjusted parking brake
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      10-10-2021, 02:17 PM   #14
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Quote:
Originally Posted by ashmostro View Post
Thanks.

In that case, the first thing I’d suggest is to ask your dealer to check the rear parking brake and potentially reset/adjust it. It might not be functioning within spec.

Good luck and let us know what turns up!
Per my last post above, I plan to bring the car in shortly. Your point is well-taken and certainly a possible suspect. I will definitely make sure to add a parking brake function check into my problem description at check-in. Thx.
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      10-10-2021, 07:22 PM   #15
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Quote:
Originally Posted by delta View Post
Quote:
Originally Posted by ashmostro View Post
Thanks.

In that case, the first thing I’d suggest is to ask your dealer to check the rear parking brake and potentially reset/adjust it. It might not be functioning within spec.

Good luck and let us know what turns up!
Per my last post above, I plan to bring the car in shortly. Your point is well-taken and certainly a possible suspect. I will definitely make sure to add a parking brake function check into my problem description at check-in. Thx.
I don't think it is the parking brake. The sound and thud and vibration are substantial enough to think it is something way bigger.

OP - my build date was March 2021.
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      10-11-2021, 12:00 PM   #16
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Quote:
Originally Posted by wesleyan92 View Post
I don't think it is the parking brake. The sound and thud and vibration are substantial enough to think it is something way bigger.

OP - my build date was March 2021.
I agree with you. I don't think the parking brake is the cause of the sound/shudder I hear, which is "something way bigger" and more powerful in nature. But, the parking brake could be the catalyst for the sound/shudder event. At least, I'm not ruling it out. I think ashmostro has a point and here's why:

Both the rear differential and the parking brake are electronic-activated. The M Active Differential is in a league all its own, with many input variables contributing to "lock" based on software and its electronic-mechanically operated multi-disc clutch. Is it possible that a premature/maladjusted parking brake is holding off the rear differential via its lock control unit from "relaxing" at ignition cut-off? Is there tension in the differential that should have been electronically "released" once the car is stopped, just prior to ignition cut-off? [Admission: Though I've worked on domestic rear differentials, I have little knowledge of how the M5's differential really operates.]

I still think the rear differential is the source of the sound/shudder at cut-off, but am unsure of whether the parking brake may be the actual initiator somehow. The fact the event doesn't happen every time throws me a little, as does the fact that the sound/shudder occurs without touching the parking brake; but, the electro-mechanical nature of both systems and a possible relationship between the two is why I couldn't ignore the possibility. Just my $.02.
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      10-11-2021, 12:17 PM   #17
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Only way to tell is diagnosis! Let us know what they find, and good luck to you both. I hope it's not a huge issue.
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      10-11-2021, 03:33 PM   #18
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Quote:
Originally Posted by ashmostro View Post
Only way to tell is diagnosis! Let us know what they find, and good luck to you both. I hope it's not a huge issue.
+1
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      10-11-2021, 04:02 PM   #19
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One more thought. It's possible an uneven adjustment of the parking brake between the two rear axles could lead to a literal differential torque upon shutdown, which could then be making the diff groan because it's not expecting any relative (and therefore, internal) motion.
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      10-11-2021, 05:08 PM   #20
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Quote:
Originally Posted by ashmostro View Post
One more thought. It's possible an uneven adjustment of the parking brake between the two rear axles could lead to a literal differential torque upon shutdown, which could then be making the diff groan because it's not expecting any relative (and therefore, internal) motion.
I can see that. I have always sensed that the abruptness and completeness of the sound/shudder is the result of a release of some kind of "stored energy" somewhere in the rear differential/axles.

Appreciate your digging into the matter. Very interested to see what the dealer will say. And I will definitely share the results.
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      10-13-2021, 06:09 PM   #21
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Good luck Delta, Interested in hearing the feedback.
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      10-31-2021, 06:31 AM   #22
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Any updates to this mystery, Delta?
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