Not obvious what plane the servo is mounted in, but typically this can also be caused by a large rudder and a tight deadband on the servo center. The servo hunts for the center position, but momentum of the rudder carries it through the center before it can correct it back. If the rudder has a balance tab, a small piece of self stick velcro near the leading edge will add enough friction to stop te oscillation.

This type of oscillation is not harmful, and typically goes away once there is some air moving over the rudder.

Brad