Engine making Hannibal Lecter "ff-ff-ff-ff" whoosh/flutter under boost

[Tangent]

Yesterday my M4 started making a fluttering/whooshing sound under boost that I can best describe as an "I ate his liver with some fava beans and a nice chianti" sound. There's no change in performance, no warning lights, and I just removed both charge pipes and they're both fine. The sound is most noticeable around 2000 rpm and it either fades away or is just drowned out once I get over 3500 rpm or so.

I removed the carbon filters months ago so I'm very familiar with the normal whistle/whoosh of the turbos and this isn't that.

Any ideas what it could be? I've still got the CPO warranty so I'll take it in if I have to but since I'm going to a track day Sunday so there's no time before and I'd rather not get out there and have to call it a day after half a session...