If you are in 2nd gear driving at say 35 MPH, depress the clutch a bit until it slips , rev the engine to say 1,500 RPM over road speed then engage ( slide ) the clutch, does the chatter occur?

Chatter can occur is oil is on the lining due to a leak. If you have access to any of the clutch, depress the peddle and spray brake cleaner on the disc / flywheel / pressure plate and see if that makes any change.

Another issue is that a CV joint can wear and spin off center causing a vibration as can a drive shaft center bearing rubber mount failure.