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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?
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10 AW on fox E90 M3 with 2 pedals, gentleman’s ride
08 AW on fox E92 M3 also 2 pedals, a bit more rowdy
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