RE: What does flutter sound like?
It's pretty easy to recognize. It is a buzzing sound as described earlier and it's generally the last thing you hear during the flight until the heaviest part of the airframe it's the ground. The wing may float down a few seconds later.
Turn the radio and reciever on and try to flex the control surfaces by hand against the force of the servo as it idles. If you have any slop or appreciable flex in the pushrods, it is possible for a trainer to suffer from flutter at extreme speeds.