Sometimes you can hear that there is friction somewhere in the wheels (back) and it sounds like grrrrrrrrrr and then it stops. The BMW dealership has looked several times at it and on one occasion they said there were some small stones in the brake compartment !!!!! A bit weird if you ask me. This has happened on the right hand side and kow it appears on the left hand side. The dealership doesn't understand it either and now I wait till it happens again before I can go back. I'm worried on such a vague "thing" that if I run out of warranty I'm in serious problems i.e I need to paid for it!