HR. I experienced such a vibration in an R44 some time ago. The cause was worn out teeter bearings. The journals that fit into the teflon couted bearings were also damaged since the journals grinded it way into the hub itself
Unfortunately I was not flying this particular bird enough to catch the wear early on the bearings. I believe the bearings have since been changed by Robinson because this was a common problem from operators that are pulling lots of torque when slinging. Go to your helicopter and carefully inspect the teeter bolt trust washers. If there is any bearing material missing between the hub and the truet washers your M/R Hub need to have it's bearings changed including the journals....
Good luck