If it is linked to the adjusting of AC controls it is probably the servo motor and/or contacts on your blend door actuator. There are a couple things to try before replacing the component or rebuilding (flipping the servo gear). In my case, I've had a successful (thus far, I'm sure it is still temporary) fix by degreasing and re-lubricating the linkages running to the motor in the passenger footwell and cleaning/lubricating the cable which leads under the hood. These are both documented on the forum and are extremely easy 5 minute processes.
Good luck! Also, remember that every time a mysterious noise is resolved two more appear so pick your battles wisely!