WARNING:- MAST ROCK/PYLON ROCK – BELL 212
This is the name given to the situation that occurs caused by inappropriate Pilot input to the controls or the effects of wind or turbulence on the aircraft flight path resulting in a violent rocking of the transmission and main rotor system. This condition is aggravated by equal and opposite AFCS input, worn mounts, bearings or dampers. Occurrence is invariably in or close to the hover. Avoidance relies on extremely smooth cyclic input at all times especially at low speeds or in the hover. Should mast rock occur it is imperative that normal flight conditions are regained as soon as possible to prevent damage to the airframe and its’ components.
I would agree with SASLESS - this is what I put in a sling load PPT Presentation about 20 years ago after having scared the **** out of myself on several occasions and having discussed the phenomena with a bunch of extremely experienced Canadian long liners. Doesn't sound the same as what you experienced!
Trog