B6 S4 Shaking Alternator
 

So I've been trying to chase down a bad noise in my s4 that I was convinced was timing chain rattle, but it didn't follow any of the normal patterns or descriptions that others provided here. I had my mechanic drive it and he hadn't heard a noise like it before. We figured out that at 2,200rpm it would start making a rattling noise, which continues for a few hundred rpms, and it would come back a little later in the rev range. We put it up on the lift with me in it and repeated the conditions, and from below he found that the alternator was shaking by quite a few millimeters and causing the sound.

Has anyone come across that before? I'm not sure how to attack this problem - whether I find a replacement alternator, replace the tensioner and check the bolts or another way. I plan to put it into service mode soon to try to locate a front valvecover/crankshaft seal oil leak (maybe a partial cause?). Any suggestions?