I am having trouble with my throttle servo(Hitec HS-81). My radio is a Futaba 3PDF. When I let go of the trigger the servo makes a grumbling noise. If I touch the servo arm the noise stops. When I move the trigger to any position and hold it the servo makes noise. If I move the rudder servo without touching the throttle the throttle servo makes noise. I tried a new servo and nothing changes. The linkage is not in a bind. On the carb I have one of those cheapie ball and socket setups and on the servo I have one of those aluminum 2-56 barrel setups where an allen screw clamps on the linkage and the barrel is held on the servo with a round push on clip. Any suggestions would be greatly appreciated! Thanks.