I can't tell from your picture, but do you have anything in the stab flutes to stiffen the coro? You might try a bamboo skewer or two if you suspect the elevator.

If your vertical is also un-stiffened, it's possible that it's doing the fluttering as well, though not likely.

How was the roll trim? When I had aileron flutter problems, it was like flying in gusty wind, the wings would very slightly rock, and sometimes the trim would drift.

But, it is possible to flutter any control surface and not have control problems.

If you are feeling brave (reckless?) then you can try to do a low hot pass close in and see if you can actually see the fluttering surface. I did that once, though it wasn't planned, it just happened that I was doing a highspeed pass when I saw the ailerons moving.

Also, double check your linkages and make sure there is no slop or movement anywhere.