A wild guess if the clutch checks out to be OK - some vehicles set up a harmonic vibration when certain conditions are just so. When I drove dry bulk tankers, the entire 40 ton vehicle would jump up and down about 1/4 inch at roughly 100 cycles per minute on perfectly smooth roads at certain speeds, as the weight distribution was a near perfect equalization between the front and rear tandems. Flat beds will often do this too when you load them just right.
Anyway, I'm wondering if the slack in the chain is jumping from the top to the bottom of the chain, and back and forth under certain conditions. If the clutch checks out, try changing your chain adjustment just a bit and see what happens.
Just a wild thought...
