Are you saying that when the heli, engine off, is sitting still on the ground, that the rudder servo is twitching (not just buzzing a bit)? If so, that is a don't fly condition and the most likely culprit, in my experience, is the servo. A servo swap is the easiest test of this and then once the twitch is resolved flight test very cautiously and low until you're confident the gyro also operates properly. Otherwise, if you don't want to flight test the gyro, your option is to send the gyro in to have it checked out.