RE: What does flutter sound like?
If you've done this more than once chances are its not flutter.
Flutter does have the distinct sound everyone talks about, but usually the airplane comes apart before its recoginized.
I've seen flutter twice, once due to to large a gap, resulted in cracked hor stab, and once due to a failed servo, no damage but sure did buzz
is your covering tight? if you have a few wrinkles that maybe what he heard.
its your plane fly how you want but if you lose your trainer you will always miss it, most guys I know still fly a trainer every once in a while.
good luck