[rotorwrench]

If it is all stock, you might have a worn journal bearing in the fan. This stuff is reproduced in decent quality and can be replaced. I track and balance rotors of all types on the helicopters and can tell you with certainty that any displacement of the fan blades can put the fan out of track and cause a vibration. Vibrations lead to premature wear and sometimes failure. Take a good look at the fan to see if it isn't already bent or deformed to any extent.