I noticed a loud rubbing noise coming out from my gf's 330xi automatic when she backed out her car. The noise comes out only in reverse gear, and gets louder as you release the footbrake more (as the car gains more velocity). Does it has something to do with the all wheel drive system that BMW used (never heard of this in the old A4 Quattro), or if it's abnormal? The car drives fine, but that sound really worries me.