If you are using JR-2721s then they are probably the problem. Sooner or later, every 2721 on the flight surfaces of my airplanes have begun chattering. Not only that, if you put an amp meter on them you will find that they are drawing over 1 amp each when doing this. I ended up replacing all of the 2721s in my Bandit, Sabre and Kingcat with 8411s. Overall, I have replaced eleven (11) JR-2721 servos with JR-8411s because of these jitters and all of the vibrations immediately disappeared. I did leave the 2721s on my nose wheel steering in one of the airplanes and it seems to be ok there. Its something about the load pressures of a flight surface that causes the 2721 to start rapidly recentering itself unsuccesfully that causes the problem.

Gordon