Answers to questions:
Never hear it when parked.
Does happen before moving if slipping the clutch on a hill.
Noise is not constant, happens for a while and then goes away.
Engine mounts in good shape, replaced recently.
Tranny mount good.
No cat, plenty of clearance around the pipe.
The clutch works well, maybe it is just something that vibrates just right when engaging the clutch... Can't figure it out. I'll try tightening anything made out of thin sheet metal and see what happens.
Thanks