RE: Is this loud sound flutter?
Two things I would do, add a pair of rubber bands, and hold the plane by the wing near the fuselage. Shake up and down, and see if the fuselage moves. If it does, add 2 more and test again. Also, it could be some loose covering, go over the covering with a heat gun carefully, you will see fairly quickly when you hit a spot that's looser than the rest, it will make a sound. Also, check all of the seams where 2 pieces of covering overlap, a loose seam can get air under it, and that can be loud. Carefully tack / shrink it down some. This can happen to planes that see changes in temp. and humidity. I have been checking mine about once a year and have to tighten up something yearly. Good luck.