There's a couple of things you can do.
1) Take it to a dealership.
2) Your brake pads are glazed and it's skidding across the surface of the rotor rather than a smooth application, causing the pad to vibrate against the inside of the calipers. Since the way BMW calipers are designed, they're open ended and floating on one side, they're more prone to producing vibrational noises especially since your pads are glazed, one way to fix it is to apply some anti-squeal compound (they're glue like sustance that hardens to a plasticky consistency) to the back of the pads and they'll be quiet, or
3) Go to a big empty parking lot. Accelerate to ~30 mph and do a couple of mild stops from 30. Then do a couple of HARD stops from 30 w/o triggering ABS. Then do a couple of mild stops from 60 (if you can), and then do a couple of really HARD stops from 60. The pads needs to be properly bedded in, so that SOME pad material gets transfered over to the rotor to ensure the most friction possible without skidding.