I hear a loud clicking noise coming from under my dash when I start the truck.
All of the controls for your heating and AC are controlled by electric actuators; they move “doors” to control the airflow and temperature. When you start your truck, all of them go through a self check; from your description, it sounds as though you have a problem with one of them. The blend door actuator is a common repair for your truck. You have two of them if you have driver, and passenger controls for temperature. I recommend having a certified technician, such as one from YourMechanic, come out to your truck's location to inspect your AC system, and recommend the best path to repair.
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