SJM55! How are ya? The only thing that hasn't been mentioned is if your linkage isn't set high enough up on the control horn. It might be safe to say that if you are 3/4 of an inch up on the control horn, the linkage geometry will be able to hold the flap steady. If you are controlling your flap from one extreme end [like with some aileron setups] and the wood is too flimsy that can contribute to the problem. Not having sealed gaps can contribute. The cause can come from just one, or several contributing factors. The buzzing sound from extreme flutter is unmistakable. immediately throttle down and land. You can disintegrate the tiny plastic teeth on the servo gears in the blink of an eye. Unless you are disciplined enough to fly below flutter speed always, it's a good idea to try the simple fixes to eliminate the problem.