join the club! i have two shuttles and both have a vibration in the tail in the same fashion as you describe.i managed to reduce the vibration in one of them by making sure that the feathering spindle was centered in the head, as this was off by a large amount from new{artf}this reduced it by a large margin.as for the other one,i believe that it is a result of the clutch not spinning true,and this i can do nothing about!you may notice that the tail vibrates worse in the wind,i have noticed this