Thank you for asking your question on JustAnswer.
The other Experts and I are working on your answer. By the way, it would help us to know:
-Do you the EATC (Electronic Automatic Temperature Control) or Manual Control?
-What else have you already tried?
Thank you again for trusting us with your problem. Please reply as soon as possible so that we can finish answering your question.
Thanks for using Just Answer, the most common cause of the problem you are having is a defective Blend door actuator, that is was is making the clicking sound. I am sending you directions on how to replace, hope this helps you out.
Removal and Installation
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Click Here for Instrument Panel Removal