1995 A6Q V6-12v Odd vibration while driving.

[beattyr2003 at cox.net]

Started noticing this about 2 weeks ago.  Low frequency vibration that feels almost like your driving over a rumble strip on the side of the highway, but not so loud or distinct.  Its much more subtle.  

I notice it at low speeds and it goes away (or I can't hear/feel it) at highway speeds (60+mph).  No change in sound when putting the clutch in or putting the car in neutral.  Sound/vibration vanishes when I turn.

This past weekend I replaced both of the rear tie rods and while I was under the car I looked over everything I could.  I did not see anything obvious other than a good deal of age on various rubber bushings in the rear suspension.  I even verified the prop shaft center bearing support had not loosened up since I had replaced the prop shaft rear CV joint back in June.

I have new tires (from May of this year) on the car.

So I am stumped and I am thinking it could be a couple possible things.

1.  Tires.  They are new, but I suppose its possible.  I haven't had any crazy manuvers that would flat spot them, so it would be some sort of wear issue with the tires themselves.  I could try putting my spare on 1 at a time to see if that makes it go away, but that only helps if its one tire.

2.  Rear wheel bearings.  I would expect those to howl and even clunk, but not vibrate?

3.  Rear diff problem?  Not sure how I would eliminate this other than someone with more knowledge than I.

4.  Someone released some gremlins near my car and they are hanging out trying to drive me insane?

Anyone got any ideas or BTDT?

Thanks!

Oh and the car is a 1995 A6 Sedan with 260,000 miles on it. 

Rob