Vibration while truck is parked and idling

[crawdad]

When I crank up my truck and it is on a fast idle (around 1500 RPM or so), I notice a vibration. I can't tell if its coming from the engine or the transmission. It's hard to explain, but I've noticed that it is not a constant vibration, but seems to be rotational related that vibrates through only a portion of the engine or tranny rotation (if that makes sense at all), i.e., on again...off again, etc. If I give it a few hundred more RPM, the vibration disappears. I don't notice it at all when I'm driving. I use to have another chevy truck that did the same thing and I never did find the cause. I'm wondering if it might be the harmonic balancer or maybe the torque converter. Anybody got any ideas what it might be or things that I need to check? Thanks.