On mine it was the hot/cold door actuator. Mine did make a click..click
noise when adjusting the knob. (a small plastic gear was broken inside
the actuator) You can see it under the dash right in the middle, it has
a small arm going from it to the door. It is white plastic about the
size of a deck of cards. I did have to remove the clove box door to get
to some of the screws out but overall an easy job.