Tick/Click Behind Glove Compartment

[sapper33]

On my 91 GMC K1500, I'm getting a very loud ticking/clicking noise fro the area directly behind the glove compartment on the passenger side of the truck.

I get it when I first turn the key in the ignition, and then again when I turn the key to off. It makes a very loud "Tick" at about 0.75-1Hz (so not super fast), and it lasts usually 20-30 seconds. Haven't heard it when the truck is running, and it once it stops, it stays stopped till I turn the key again.

The truck has been running great, and there have been no problem starting it. But that tick keeps getting louder, to the point my it can be heard from street to the front door. Any thoughts?

[93GMCSierra]

that is your electric servo motor that runs the heater controls, I had the same problem in my 93 when I bought it, what happens is the plastic gear strips off and when the motor tries to turn it it makes that noise. It is the outside air control, if you press max a/c you will probably hear it too.