Hi
I'm experiencing the same issue with the nasty sound when braking and reversing. My car has been to the dealer 4 times and each time they say they cant diagnose because it doesn't make the sound when its with them. I've also taken a video and shown it to them but still nothing. I don't have an issue with burning breaks though.
I'm made a complaint to BMW UK and am now waiting for their response. Incredibly frustrating! Any advice??