Antman:
Flutter happens at higher speed. Sounds like something is resonatingwith the engine, as you say, at a certain rpm. maybe a pushrod hitting the side of a former, two pushrods hitting together, covering vibrating. See if you can duplicate it by having someone hold the plane at the same attitude, and open the engine up to what you think is the same RPM. If it does, go around and touch the surfaces and see if you can get it to stop.