Warren,  Maybe it's the music roll that's playing, "I Wish I Could
Shimmy Like My Sister Kate!"

Okay, seriously now, is the flywheel (the big pulley) out of balance?
I don't know if these were balanced with the crankshaft, or just by
themselves.  If you took the pulley and crank to an engine balancer,
they could check it out and maybe improve the balance.

David Dewey