I have the same issue. I brake at a light, speed bump, stop sign, etc. and then when I let my foot off the brake and the car starts to accelerate there is a squeaking sound that sounds like its coming from the rear suspension. It goes away above 10 or 15 mph. It doesn't happen when I have the brakes applied so I thought it was the suspension. Brought it to bmw a few times and they cannot duplicate it and cannot resolve it. It's very frustrating. For some reason it doesn't happen all the time, it comes and goes. I find it amazing that I spend a lot of money on a BMW and I have to deal with this sort of crap...a squeaky car.
So legally how can BMW not fix the problem when you have a Bumper to bumper warranty? Does it say something in the warranty documentation about this?