The best way I can describe it is this - my engine lightly shakes around 1000 RPM or so (I'm guessing because I don't have a tach). It doesn't make any extra noise, and it's fairly light shaking. It feels just like the vibration from driving on off-road/winter tires, but I know it's the engine and not the tires or the driveshaft because it does this even when it's in park if I give it a very slight push on the gas. Could it be the MAP sensor? Harmonic balancer? Radiator fan off-balance? Broken motor mount? Bad timing?