RE: What does flutter sound like?
Like that definition that Jim posted, it's usually a low-pitched buzz. As far as isolating it, that's not really possible because it's not something that you want to produce on purpose. Trust me, if you had flutter you probably will know about it before too long as it can literally destroy a plane withing seconds. If you think you have flutter on your plane you should land IMMEDIATELY and investigate for the cause of the flutter. You should not fly the plane again until you figure out what was causing it.
Ken