This quote in reference to 5-Volt PS2 boards "chuffing in neutral" is from the MTH Service Notes. I wonder whether the problem has come back from the dead.
PS2 Steam Chuffing in Neutral
We have found that some models exhibit chuffing in neutral. To eliminate this problem place a 10K
ohm resistor in line with the orange wire going to the tach reader. This applies to 5 volt Ps2
equipped models only. The 3 volt Ps2 Board has the 10K ohm resistor on the board.