Any Thoughts on the Likely Cause of this Noise & Vibration?

[Marc Boucher]

Drove about 1/2 hour on the highway tonight.  On the return, noticed a noise, along with a slight vibration when first entered the highway.  Since noise hadn't been there on the 1/2 hour drive before, figured it was just a rough road surface.  But after a few minutes of this, realized it was the car.

Felt almost like a tire was bad.  

Realize this could be many things...but here's one possible clue.  When driving at highway speed the noise/vibration is small, but noticeable.  When I take my foot off the gas pedal and let it coast for a few seconds, the vibration intensifies even though the car is actually slowing down.  Finally, when exiting the highway, even thought the speed is slower, the vibration once again intensifies as the car enters the curve of the exit ramp.

Second clue, there was no vibration in the steering column, it was more throughout the car.  Car steers dead straight, doesn't pull whatsoever to either side.

Car is 1990 Audi 100, not a quattro.

Any thoughts?