Rodney

First off, this is not flutter, at least not in convention definition of flutter. Instead it is an oscillation which can be caused by several factors. The more probable in this case is a slightly sticky push rod that has a lot of friction to movement near the center position. It could also be caused by the rudder hinges not being in perfect alignment causing them to be at rest in the not-quite-neutral position and wanting to snap to a slightly off set position an neutral. It could also be a dirty potentiometer in the servo itself or a well worn one. If it is the servo, you should be able to see the servo jitter when the rudder push rod is disconnected from the servo. If, with the rudder push rod disconnected from the servo, the servo does not jitter then it has to be friction in either the push rod or hinges on the rudder and the servo has a clean bill of health.