Antman, what you are hearing is not flutter. Most likely it's harmonics amplified by the big thck wing and wide rib placement. Causes could be many. Loose covering, crackd rib, loose glue joint, etc. or none of the above and nothing may be wrong. Have you ever heard the sound a pizazz makes when placed in a hover or you should hear my UCD46 with a Saito 100 rattle. Check it out for loose stuff, fix any you find, if all looks OK just go fly the thing and have a blast.

If you ever hear a plane fly over and flutter starts, you will hear it and most likely recognise it, you just won't hear it for long.....

Ed M.