Chosen Solution
Heater and A/C both work fine in the vehicle when using internal air, but when I want to defog the window or bring in outside air I get a rapid clicking noise coming from under the dash. Sounds like there is a switch that is trying to click open but is stuck, possibly something with the divider gate that switches where the airflow is coming from. Not sure how to better diagnose or how to get into the dash though.
Ethan, My dad had a very similar problem with his Silverado. As we learned, there are actuators within the dashboard that control the positions of the “blend doors”. These doors control the amount of AC air that is mixed with heater air to achieve the desired cab temperature. The actuators are controlled by the car computer. The issue, at least in my dad’s case, was that the alignment of the doors was off, so that when the temperature was set to max cold, the door hit the stops and could not move any farther. However, the actuator kept trying to move it, causing the clicking noise. I know there is a way to adjust the actuators/computer to correct the alignment, but I’m not sure how to do this, and it obviously differs from car to car. Also, just a heads up, the blend door on my dad’s truck eventually broke off, and the repair is much more difficult once this happens. Hope it helps!
