Go to different dealership.
It's well known problem.
Your rear brake need to be fixed.
Search for brake squeal.
E92 M3 Dakar Yellow DCT | CF Trim
Akrapovic EVO Exhaust | RECARO RS-F SK2 Seat | KW Clubsports | H&R Sway Bar | DCT Cooler | iTG Air Filter | Volk TE37 SL + RS3 Tire