I think probably what you are seeing is merely the power pulses reactions at low speed causing the whole airframe to skake. I'll bet as you increase the throtle from idle it smooths out. If this is the case it is something that just happens generally at low speed and there is not much can be done. The power pulses plus the compression reaction at these slow speeds will cause the airframe to shak considerably. I don't look at them as vibration, butjust a shake that must be endured till rpm's are increased to begin taxiing when it should go away.