Hi Collective B',
The scenario you stated is exacltly what you suggested - Sprag mounts. This is a fairly normal indication of worn sprags. Most engineers etc refer to the motion as a 'shunting' one per vibration. You may or may not know that it is normaly only the lateral sprags that give you the indication and it is more apparent in KH4's than in B47's, (Although that is a generalised statement). The sprag settup is different in a KH4 than it is in a B47. The fore & aft sprags, for example, are both mounted at the rear of the sump in the Kawasaki, this can give you a slight rolling feel to the shunting.
Engineers will either replace the sprags as a set or change the tention on the nut and redundency plate (washer),this will alter the frequency of the rubbers.
As part of your daily, on a skid mounted machine, try rocking the mast from side to side, the easiest way is to have the blades in the lateral position and use their inertia to rock the mast (don't be a woosy about it, nothing will break) if the mast returns to the centre position in a mushy manner, the sprags are stuffed. The correct sprag indication should be a return of the mast to the centre position in a short sharp oscilation. The amplitude of the rocking should reduce rapidly with an almost jerking motion to the centre. It should not rock back and forth evenly like a spring.
Hope this helps out.
Cheers mate