Originally Posted by mike_o
Be careful not to blame the mounts too quickly - The vibration has to come from somewhere.
First of all, make sure the props are balanced. If you are still experiencing vibrations, try rotating the prop in relation to the motor by 30 degrees at the time until best setting.
When you have achieved a reasonably balanced motor/prop combo, then you can look into the mounts; I have secured the front part of the mount with hot glue, which can be removed if need be. Furthermore, I have added a lower foam bridge between the open, lower nacelle halfs. Also hot-glued in place. Runs very, very smoothly now
And do NOT fly with the described vibrations!
Harmonic resonance is something that affects EDF. The advice is to avoid the power settings that bring it on. Also, I think he said he balanced the props.
If he's balanced the props, stiffened the mounts and avoids the affected power settings, then he's done all he can do.