Ian.
Don't be under the impression that flutter can only occur at high speed. I've experienced it myself whilst test flying a Synergy 3D for a clubmate. Occured at less than half throttle.
Flutter normally is a low pitch noise, but can get very load if you don't "close the taps" and slow the model down.
Things I'd check would be;
Slop in control surfaces
Engine/mount secure
Prop balanced and not damaged, i.e tips
Anything else, like U/C etc. that could resonate.

Good Luck.

Ian.