Last weekend while reversing out my E90 from the porch, I hear klak klak sound from the engine, so loud that it's almost same like FWD car turning with a critical stage busted drive shaft. The sound stop after that but ever since, I can feel vibration in the cabin, even in the shift knob. When on aircond or engage gear, the growling sound a tad louder compare to neutral with aircond off.
I just replaced the alternator bearing and vBelt tensioner plastic pulley.. now car okay d. It seems BlackE90 replaces the same items sometimes ago. The alternator bearing is like our bicycle rear sprocket. It allows one way spin together with the belt but when engine slows down, the alternator's spin momentum will continue to spin despite vbelt slowing down, hence reduce engine drag or lost of engine efficiency.. fantastic innovation but all in, to replace both metal and plastic pulley, I spend RM420..
Btw, my mileage only 56k km, less than 4 year old N46 E90 320i.
This is the alternator with pulley taken out. The plastic tensioner pulley on the left of the alternator also I ask to change because got free play liao..
This the alternator pulley.. there's a catch and release mechanism inside the pulley, which mine can catch but can't release, thus transmitting vibration to the cabin.. check out the tools require to open this bearing.. some say its "special"..
I'm not sure if this helps.. just to share my experience..