Chuging as Robinson puts it is PYLON WHIRL it is common with Bell style two bladed rotor systems . one blade flaps up and the other flaps down as a single unit.
As the blades flap to releave disemitry of lift. they also see different air loads. different loads in mass & drag so some times the pylon will turn not in line of the turning shaft but turn in a circle around the plyon with the shaft whipping from aerodynamic loads that are out of ballance in the plane of rotation, the effects are minimized by design with the rotor blade inplane stiffness & gearbox dampers. If the rubber in the R-44 is too soft & the C/G is a little out of limits the pylon could shake the tail off .
Look up pylon whirl , Hooks effect , Aeroelastic .