It's the sound your plane makes just before it disintegrates in mid-air. You can avoid flutter by taping your hinge-lines or MANAGING YOUR THROTTLE. Either way is sufficient. I do both.

I have literally fluttered a plane in-half. Just remember that a trainer is not likely to attain flutter speds, no matter what engine you can FLY it with. If your plane were fluttering, you would surely know it, and this thread would never have occured.