Chevrolet Repair Questions? Ask a Mechanic for Answers ASAP
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THis is commonly caused by a failed a/c temperature actuator. When the actuators internal sensor fails, the actuator will often move back and forth and "hunt" for the correct position. Which side of the dash was it blowing hot on? The left side is relatively easy however if it is the passenger side, it requires dash removal to access.
I was asking which side of the dash was blowing hot... the left or the right side?
There are several actuators in the dash however your left side has failed, should be replaced, and is likely the noise as well. Remove the screws from the right side under dash hush panel if your truck has one. The actuator is on the bottom near the center of the hvac case. There are two 5.5mm screws that hold it on. You can also put your hand on the actuator when the noise is present and feel if that is the actuator making the noise.
Hopefully it was the same actuator making the noise as causing the hot air on the drivers side.