A/C Popping under dash

[rt4x4]

I have an 2004 silverado ext cab with the manual dual zone a/c and heater controls. when i sart up with recirculate mode there is a popping under the dash,when i move the thermostat levers to heat andback to cool it stops.I was wondering what caused this and what needed to be fixed, truck is almost out of warrenty. thanks

[ChevyTech007]

It has a bad HVAC actuator. Get it fixed before it goes out of warranty.