Common problem and I think somewhat easy fix. There are 3 actuators on
the Impala. One on the left, the right, and a blend actuator in the middle.
They’re made completely out of plastic and the clicking sound is the worn
plastic gear trying to mesh with the heater door. 2 are the same actuator but
the blend one is larger. If it’s coming from the right you can pop out the
glovebox and you hear it and see it. Not difficult at all especially if you have
smaller hands. Changed all 3 out THREE times with Dorman but the 3rd
time I used AC Delco. Buy them at the auto parts store, they’re about the
same price as the aftermarket. Haven’t had one fail in 2 years.