[fox46]

Describing how bad this jitter is can help provide a more complete diagnosis. Is it just a jittering sound or how many degrees of movement are you seeing in either direction? Servos will sometimes hunt or occasionally "buzz" when trying to find their center.

My bet would be that since these kinds of servos have a definite lifespan and are not particularily high quality, the potentiometer inside them can develop wear, particularily around the center of their travel (home position) as this is where the servo spends most of its time. If this servo has been used previously it is possible that the potentiometer has become worn and thus the electronics are "hunting" trying to find the home position or center. If this is a new servo, it is possible that you just have a defective unit. Try adjusting the center position in the code so that it is no longer in it's current spot. This should move the potentiometer onto a less-used or "good" portion of its travel thus hopefully providing a cleaner feedback signal to the controller which will stop it from hunting. I have seen servos develop these symptoms quite a bit with radio controlled aircraft and cars as they can often experience a fair bit of mechanical vibration during use which wears the potentiometer very quickly.

Try swapping a new servo (preferrably one that is brand new out of the box)in its place and see if the problem is still present.