Pitt, I don't know about your timed-rotor helicopter. But two-blade rotors can exhibit vibration at various stages of cyclic pitch, particularly in banked turns. And especially if they are being run too slow (low headspeed). I don't know if it is the disturbed air from the lift pulse of the blade, or simply the noise. But I sort of think that in most cases it is the noise, or sound waves, causing high vibrations. The "bark" from helicopter blades is considerably more pronouced than the bumble-bee type "buzz" of fast-turning multi-rotor blades.
I have always had better results mounting the camera underneath vs in front or off to the side. Especially in high-speed flight. The camera in front screws up the sleek aerodynamics of the canopy/cowling and causes its own problems with wind load on a quite blunt suface - underneath not so much. The Trex 800 "Trekker" with that big contraption hanging out in front was probably one of the worst-designed unmanned flying camera platforms known to mankind - and probably why it wasn't very successful in the sales dept.
Solid mounted camera works by far the best on helicopters at high speed. Gimbals become totally worthless at flight speeds over about 35-40 mph because they simply can't take the wind load flying upwind at airspeeds of 60+ mph.