Since it is only on the rudder servo and only when the engine is running, it is almost certainly a connection problem in the rudder circuitry. It could be at the receiver where the connector plugs into the receiver or anywhere between the receiver and the electonics within the servo. First, plut the rudder servo into one of the known good channels and see if it continues to act up with the engine running. If not, you have a problem in the receiver itself. If it still acts up, you have a connector problem in the harness to the servo or within the servo itself. Change out servos, if the problem still exists it is in the harness, if not you have a bad servo.