Update: OK this is weird!

Driving on the freeway, the vibration was getting worse up to about 60 MPH, then stayed the same until about 70, then all of a sudden, while I was cruising at about 70, there was sort of a little clunk and the noise stopped!

After that, the noise stayed gone for 5 minutes or so, while I cruised between 70 and 75 MPH. I tried accelerating, engine braking, etc. and the noise was staying gone. However, the gearshifter was shaking back and forth more than usual, sort of wobbling around rythmically.

Then after a few minutes, the noise came back <img src="/forums/images/graemlins/frown.gif" alt="" />

Doesn't this make it sound like there is a problem with the gearbox? Maybe some bad bearing somewhere that sometimes pops in and out of place or something?