I beleve flutter is not always detectible by either sound or visually. This brings to mind the classic cases where a single hinge or more hinges fail continually frequently on ailerons and no once see's or hears it. Thats why the hinge type or hingeing workmanship is blamed when in fact its flutter.

How about this classic the aileron servo out there in its box just keeps mysteriously losing its mounting screws. Worse found just flopping around in the wing but no one heard or saw a thing. Throw a mass balance on it and just as mysteriously the screws stay in place no problem

John