odd vibration

[IggDawg]

Just about the only wierd thing my engine does is this part throttle vibration thingy. It only appears under very specific conditions. If I'm cruising on the highway or anywhere at a constant speed, and applying just a tiny bit of throttle, I get this weak "rumble" from the engine. I want to say it feels like timing is being pulled. if I let off on the throttle or go deeper into the throttle, it goes away completely. she purrs like a kitten at idle, and sings beautifully and smoothly at WOT up to the redline. it's just at this part throttle condition that I get the rumble.

any clues? is this the "lean condition" thing I keep reading about? I hope the ECU isn't bottoming out my timing and still looking for more. that'd mean I'm prolly getting some ping and not knowing it.