Okay this problem really has me baffled. My wife has been complaining of a clicking noise in what she thinks is her A/C vents when on the interstate doing 70 or faster. I thought she was crazy so I drove it to see for myself. Sure enough there is a very apparent "clicking" noise coming from either the A/C or behind the dash somewhere. Here's the kicker though. It only happens when behind another car or truck at 70mph or greater AND the noise will shift from the left of the dash to the right and then back to the left. Very very strange. Going 70 or faster on the interstate with no one in front of me it's fine, its just when I have a vehicle in front. I've noticed the larger the truck in front of me the louder the noise is. Any ideas? Its driving my wife nuts having to deal with it. I've checked for leaves or pinestraw under the hood but its clean as a whistle.

 

Maybe something with the sunroof drain?

Also try changing the Temp on the climate system and see if it makes a Difference.