Do you only hear it on decel? The alternator has a one way clutch on the N54's that lets it freewheel when the motor spools down quick. Put the car in neutral and rev it to redline then let it come back to idle quickly. You'll hear the alternator wind down slowly.
If it's not that, figure out if it's engine speed dependent or vehicle speed dependent. Does it come through the radio speakers?
2011 335is coupe white/red
06 kawasaki ZX6R in Kawi green