Random vibration

Truck is an 01, pee pumped, 5 speed, 2wd. Was in traffic a few days ago, and got a vibration out of nowhere. Was sitting still, truck at idle in neutral. Felt the vibration through the seat in my butt. A pretty noticable vibration. It's done it 3 times. Never lost oil pressure. Idle speed didn't change. Went over everything, I think. Thought exhaust or ic pipe may have been touching something . Engine mounts seem fine. All pulleys and belt seem fine. Could the harmonic balancer be taking a crap? Truck has 306k. As far as I know, balancer is original.

Clutch?

I don't know. It's done it with the clutch depressed and released. It's got a valair single ceramic clutch in it.