hi Oldornet and Nick,
This 76 is now under a programmed maintenance, therefore we have stopped tracking this vibration for a while.
After having changed and retuned the MTA as well as removed, inspected and re-installed the biffilar, we have noted some improvement but the cycle of vibrations is still here.
In the past, we had solved a vibration problem almost similar to this one in changing the pilots hub (removed the old version to the new one with larger edges)
We were not expecting this improvement, in performing this operation but it worked. It is difficult still to find a relationship.
Of course we have tried to fly without DAFCS at all, just in case, but no change.
As you say Oldornet, the helicopter is serviceable but it is annoying....