I had this problem with a Fox 35 powered Gieseke Nobler a few years ago. Ran Superfuel and a Top Flite 10x6 Super M prop. With the first test flights, I thought it worked great, until it would hit resonance every now and then in level flight. It would rattle so hard I thought it might disintegrate, then, the fuel delivery to the engine would be disturbed, the rpm would change, the resonance would stop, the engine would return to its normal rpm, and the cycle would (might) repeat. After two or three flights with this alarming issue, I "hid" from the problem by using a 10x5 Super M and running some more rpm. The problem never came back, but the airplane suffered a major structural failure only 100 flights later, something I attribute to the vibration it was exposed to. Resonance can be very destructive...