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Q: AC causing knocking sound.

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The AC and heater will work sometimes on all levels on the switch. Both will produce hot and cold air. But then the AC/heater system will not work at all. If my truck sits a while, the system will work. When I start the AC, I can hear a knocking sound from under the dash.

A: The knocking noise is the temperature blend...

The knocking noise is the temperature blend door actuator trying to move the blend door. The actuator would need to be checked and replaced. I recommend having a certified mechanic, such as one from YourMechanic, come and inspect your vehicle's AC system for you. They will be able to repair or replace any of the components to get your AC functioning like normal again.

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