Twice I have seen two identical planes, one with flutter and the other without. And they had different causes. One plane had sharpened trailing edges on the aileron VS blunt, the sharpened aileron fluttered. When the aileron was replaced with blunt TE, it stop fluttering (the replacement was actually softer lighter balsa!). Another had thinner ailerons then another plane. The plane with the thicker stock did not flutter (it even had a larger hinge gap). The leading edge of that aileron was a bit thicker than the trailing wing edge just ahead of it. Thin balsa veneers were applied to the aileron sides on the plane that had the thin ones and the problem was again solved. I'm guessing this kept the control surface in contact with the air flow. Linkage and servos were tight in all cases.

If you hear flutter, dump throttle, pull up to dump speed and load control surfaces, land and fix.