Balls and Dutch Roll
I thought that it may be a sticky vibration damper (balls) and I got the technicians to check them but all were ok. One engineer here said that it was the resin at the blade root breaking down allowing the blade to move within its root sleeve where it blots onto the head. Maybe?
I also thought about Dutch Roll and remember my old ground school instructor telling us about it during flying training at RAF Leeming - he was a Halifax pilot during the war. But dont recall how it happens in a Halifax let alone a helicopter. Also this particular helicopter was really bad compared to the others where it was hardly noticeable at any speed. They all tend to 'nod' a bit if you look hard but it is usually not noticeable although more so at low speed.
The porpoises along in pitch - actually a very similar effect only much less pronounced but more noticeable if you sit in the back.
Thanks you for the replies.