HeliTB I agree with what you are saying but I have confirmed the main shaft, axle shaft and motor shaft are ok. When I replaced the belt I looked inside the tail boom and made sure the belt was not crossed or twisted. The new belt did not help. I have ran the tail belt tight enough to produce gear whine and I have tried it loose enough that the belt almost slips and almost everywhere in between. In both cases the vibration was there but it changed as to what rpm the vibration happens. At lower rpms there is no vibration. It is amplified when the rear blades are installed.

Thanks,
Astra