Did you get the flywheel balanced after it was lightened? Because the flywheel acts as a dampener, just like a harmonic balancer.Take some of the mass away from the flywheel & it'll let the engine buzz.
It could also be that the flywheel is now itself so out of balance that it is the cause. You could either pull that flywheel out & have it balanced, or get another one lightened then balanced, & swap them over.