However at 2,450 and above a vibration starts and up to 2,850 redline, it's quite bad, the higher the RPM the more vibrations present.
mmm, perhaps a jammed crankshaft counter weight.
I would suggest put another accelerometer(velocometer) at the rear of the engine and check vibe level as you lower the forward vibe level.
Have had same issue with PT6A in Twin Otter ( I know is turbine) and was an engine vibe problem - found with the rear accelerometer.
Cheers
BH