Droning Sound...

[Chicayne]

My buddy's e46 330i - droning sound when he applies brakes (when reversing only). Any ideas what the fault/issue could be?

 

[Waseem 333i]

My one does that also after i wash the car. The rust build up causes it i think. Also ask him to check the wear on the pads. Could be its almost worn out. Mine is due replacement soon.