It's not a high speed vibration like your chopper? Make sure it doesn't vibrate, it'll tear your machine apart. We had one of our machine vibrate on the lower variable drive sheave also, it broke loose on the frame. I wound up cutting the channel out and installing a one inch plate drilled and tapped to fit the spindle. I've changed the bearings out twice in there, they are difficult to balance. I need to get a service manual for those machines, I think the sheave halves need to be timed.
Maybe you should check your fan bearings, try changing speeds on different components and see if the frequency of the vibration changes.
Good luck.