This sounds like a well known problem with GM and Ford HAVC systems. Where they used to use vac servos to move the blend doors they now use stepper motors. They are easy to install (once you get to them, that is) and take up a lot less room than a servo.

Normally they will make a clicking or thunking sound and on GM's will do it even if the key is off.

There is no repair; the one that is bad just has to be replaced. I think they run about $70 each for parts.

The good news is that it will not get any worse. The bad news is that some of them are very difficult to get to.