Garmin Class A Mechanical Autopilot Random Vibration

[IronZebra]

I have autopilot that uses a Garmin Class A Mechanical Drive (no hydraulics). Specially it is model A1-0177. It works great and have no trouble with it while underway. But after it is shut down it will randomly (once or twice a night) make this horrible vibrating noise that sounds like” ber rerrrrrrrrrrrrr” then stop. Next day no problem and works fine.

It is weird that it only happens when shut down. I did not think it even had any power going to it after I shut it down. Any one have insight on what is going on?