Do a good range check with the engine running.
Put a rubber band on the linkage to load it down a little.
That will dampen the vibrations and let you know what might the
cause.
I would double check the rec. The crystal might be loose in it's socket.

Sometimes a servo will get a bad spot on the pot at the center of it's trim and buzz a lot. I see this on my glider a lot.
Servos will also buzz if they are under a load too.
Worse, is my HiTec gear will buzz a servo when the transmitter is too close to the plane. Move back 10 feet and it's fine!!
Rich<>