RE: What does flutter sound like?
After enough description covering the sound of flutter I'd like to add one thing. Taking for granted a reasonable linkage, sealing the hinge gaps with a piece of clear Oracover/Monokote/whatever brand you like is the single best anti-flutter device that takes about one minute per control surface to install.
A couple of experiences of mine. I had an Arrow, ~1980 patternship, fast flyer that had too soft torque rods and unsealed but very closed gap airlerons. First flight saw one aileron flutter and get ripped off, hanging by the torque rod. I managed to save the airplane, fixed the aileron (not the torque rods), sealed the gaps the way I should have in the first place, and never had flutter again.
A couple of weeks ago, I was flying a Synergy 90 3D and it had been twitching along the longitudinal axis (that's the roll axis) as if I had a defunct servo. I flew a few minutes, fast, slow, up and down, and after landing discovered that an aileron hard point had come loose inside of the wing. If any airplane will develope flutter, its an airplane with oversized control surfaces. I had properly sealed all hinge gaps prior to ever flying the airplane (its been a standard part of my building since the Arrow incident) and can say that it was that tiny strip of Oracover that kept the aileron from fluttering the airplane apart. I experienced no flutter even at high speed with a floppy aileron.
Lastly, sealing hinge gaps really helps aileron and elevator response and makes for a much more trimmable airplane. I even sealed the gaps on an Astrotrainer so that my guests have a more honest airplane to try (I like to share my hobby with friends). A roll of clear covering is enough to seal the gaps of a couple dozen airplanes.
Mark