Yup, Tom and Kyle are correct.
The only perfectlly balanced engine is an I6 (well, technically flat 12, haven't seen one of those in a car) where every impulse occurs with 1 piston at TDC, 1 at BDC, 2 going up and 2 going down (i.e. 0, 60, 120, 180, 240, 300 degrees) which is why (semi) truck engines and the huge boat engines are all 6 cyls. In an I5, the number of pistons moving up and down depends on which piston is firing (odd number). If more are going down then up, this jerks the crankshaft down a small amount (and vise versa). When these impulses occur at a frequency such that the the jerks reinforce each other (resonance) they can't all be absorbed by the motor mounts and the engine vibrates. This happens usually around 2k (35hz) and 4.2k (70hz) for the T5.
It's just one of those cooky things about the B5254 (along with the tappet noise, lol). If you've been around them long enough, you can tell if it's firing on all cyls by the sound reving from 1.5-2.5k
