Noise from AC and Defrost

We have a 2002 Trailblazer, recently purchased. It has just started doing something strange. When you turn the ac selection to either just feet, defrost/feet or defrost it makes a clicking sound that lasts for a few seconds. It then stops then starts then stops and keeps repeating this process. The AC and defrost work otherwise but noise is annoying. Any ideas and possible cure and cost?
Thanks

Mine has a similar problem. It's a 2002 LT and every morning when I first start it, the air conditioner makes a clicking sound for about 3 seconds. It used to just do this but now will continue to click for 10-15 minutes after I first start it but not as loud. It will also stop blowing cool air at times (normally when I'm idilling for a while) and sometimes the outside temperature will read 50 degrees when it's 90 outside. When the AC starts cooling again the temperature reads correctly. I think a post some time ago said it may be the "AC accuator cam". Any ideas out there?????