Engine Vibration-1957 Series 62-Rebuilt 365ci

[CORVAIRWILD]

I just bot this 1957 series 62, the engine is reportedly rebuilt, and looks and runs like it is.  Unfortunately it vibrates at idle and somewhat goes away until about 40-50-60mph. It's really annoying,  just droning and droning on as I cruise along.

I'm a mechanic, so am familiar with the various causes of vibration... Defective torque converter or out of balance,  but its not the driveshaft or out of balance tire.

I'm going to ask the seller for the engine rebuild invoice

[bctexas]

I had a vibration in a small block Chevy once and finally tracked it down to a defective harmonic balancer, if you haven't already checked that.  From the parts book it looks like the '57 has one.....

[CORVAIRWILD]

I haven't looked, and the car is currently parked 300 miles away. I'll be able to look Sunday