If it is under turning and or acceleration, have you made sure you're not rubbing your fender? As far as the diff goes BMW says that its lifetime for the oil but in reality it should be changed after some time. You might try finding some redline or similar to put in as the BMW stuff you will have to get from a dealer. You can also look for some redline or royal purple diff oil i think you would be looking for a 75W90, it might be slightly more but I know many who swear by these fluids.