My guess is a warped rotor.Do you feel any pulsation while braking, especially hard braking to slow down from highway speed? It could also be a loose caliper. Do these cars have any rubber hardware in the caliper mounts that could be shot? If so, this and a loose caliper coupled with a warped rotor would make a good bit of noise/thumping.
Kevin in Hillsboro Oregon1973 220D
