Originally Posted by Tacco
Could it be that I just hit some kind of resonant vibration frequency of the flap handle itself?
That would be my guess. I had a similar issue in a Piper Warrior that I owned. On short final, at a certain rpm under certain conditions, the door handle would suddenly come alive and start vibrating like crazy! It would only happen occasionally, but when it did, it was loud enough to be quite startling. I tried to track down what was causing it, but was unsuccessful.