Bad vibrations

[Michael McLaughlin]

My '99 A4 is in the shop for a vibration. I initially thought it was a 
wheel bearing (the symptoms were similar to the one on the other side 
that was replaced earlier this year) but I just got off the phone with 
my mechanic, who said they're not sure it's the bearing and don't 
really want to replace it if they're not sure that's the problem. His 
advice is to bring it back when it's worse and therefore easier to 
diagnose.

Here are the symptoms. There's a vibration up to about 25 mph, where it 
more or less goes away. Above 25 and below 65, the vibration is very 
slight -- if there's no one in the passenger seat it'll vibrate a bit, 
but that's about it. Above 65 mph, it comes back with a vengeance -- 
when passing on the highway the car shakes pretty badly. In hindsight 
it doesn't change when turning like the other bearing did, and it's not 
nearly as noisy, so I think my mechanic may be right that it's not the 
bearing.  The one thing that made me think bearing is that I've been 
getting some odd tire wear that looked like the odd tire wear I got 
when the other bearing went bad.

In the meantime, any ideas on what other possible causes might be? The 
car has been aligned recently.

Thanks,
Mike