Broken diode is the usual cause of your symptom.

Alternators generate three phase electric AC and that's converted to dC though a 6 diode bridge rectifier.

When one or more of the diodes fail (usually open), you'll get lumpy 2 phase power. This will give bumpy torque at the drive belt causing vibration.

It can also cause electrical interference that you can hear in the stereo.

__________________
2011 E70 • N55 (me)
2012 E70 • N63 (wife)