Karl Townsend wrote:

Pete and Don, I also agree. I've said it CAN'T be the fan vibration for
years. But I also can't find anything else wrong. And its not for a lack of
trying. I'm about to pop $11,100 and trade it in for a new used one.
Julie's not to keen on that idea, but only because we haven't got the money.
Got a big electric motor? Can you borrow a VFD off something for an
hour? It might be REALLY instructive to spin the thing up on electric
power and see if the vibration shows up, or not. If you can't detect
any vibration under electric power as you vary the RPM across the
troublesome range, then I think that proves it is NOT an out-of-balance
condition. That leaves speed pulsation from the IC engine as the cause
of the problem. Some kind of coupling that softens the pulsations would
be the fix. When the engine puts out these pulses, it slams the gear
train from one side of the teeth to the other, and this is REALLY hard
on all the parts. I still think that is what is going on in this case.

Jon