Vibration caused by; loose engine mount, loose engine to mount connection metal pushrods vibrating, loose muffler, ect. ect.
Try tapping on all the various parts of the plane with the plastic handle of a screw driver, and see if anything makes a "loose" sound (rattling instead if solid). Check all nuts, bolts, screws, ect. Make sure your Rx is isolated well, and double check to be sure the crystal is fully plugged into thr Rx.

Jetts