Get the engineer to check the T &B a few times in between a few flights up to 75 KIAS and ensure the same result is achieved without any adjustments.
I had an increasing vibration over a period of a week and as I was the only instructor flying the helicopter I could account for it not been oversped etc. Then on one flight half way through the student making a steep turn there was an unusual increase in vibration etc and the cyclic felt different. I made a precautionary landing and when I could not find an issue and having discussed with the engineer I flew back to base at 60 KIAS without incident. A full check was carried out and we concluded the T & B was within limits but decided to make it as smooth as possible so made an adjustment. After a few quick flights and small adjustments we found our selves making changes but with no improvement and the plot moving very randomly around the graph. After 4 moves the engineer said he thought he knew what the problem was - we checked the flapping hinge tension which whilst within limits did not feel right so we pulled the pins and sure enough the bushes were very stuffed. We replaced these, tracked it up and it has flown 600 trouble free hours since. The worn bush has clealry moved under load but the vibration was not felt until above 65 to 70 KIAS.