Vibrations...

I would try putting the rear on jack stands, as an earlier post said, and run it through the gears. If you're not careless you could also try doing that with the rear wheels off of the car as well just to verify. If you have front discs, you may as well check the brake rotors for flatness. I've also heard of water pump and alternator bearings going bad/warping the pulley shafts. So maybe try isolating the accessory drive as well...