Irritating and frustrating. I've been there, on most of my rides, though ever so briefly with my LCI M5. (The night is young--I only have 2800 miles.) Comes with the territory of owning a performance vehicle I guess.
In the end, when I first heard "the squeal," I usually tried a few aggressive stops (including in reverse) to see if I could break up any dust build-up or glazing. That worked, but often only temporarily. Sometimes, I took off the brake pads and coated them with products designed to quiet pad vibration, e.g., CRC, Locktite, etc. Perhaps BMW has its own "proprietary" product for same. In most cases, that solution did work.
But what
always worked--changing pads. Didn't do it if the squeal wasn't unbearable and I was able to get used to it (especially if the sound didn't occur every time I applied the brakes). But the fix was a pretty solid one. Don't know how the dealer looks at that solution though, what with your M5 still under warranty.
Oh, check out this thread if you haven't seen already:
https://f90.bimmerpost.com/forums/sh...1515537&page=2
Good luck.