It is unlikely that the TT shaft is causing a vibration. If it is a vibration when just spooling up, but goes away when you get to your preferred head speed, then that is completely normal and nothing to worry about. But if it is a constant shake, then double and triple check your main shaft, feathering shaft, tail shaft, blade tracking, blade grips (front and rear), feathering shaft bolts, tail block that hold the blade grips and so on. Sometimes it is hard to find a vibration, but it is always worth looking into until you do find it.