My manual hvac in the truck is acting up. The fresh air/recirc select is making a loud clack sound when it operates. All the other selections work fine throughout the ranges, upper, feet, both, defrost, etc. The blend door functions properly, fan selector works properly. I have noticed recently with the window down air flows out of the vents like the fan is on. Any ideas what could be causing that? Could it be a byproduct of the noise I'm hearing with the fresh air/recirc clack noise I hear when I make the change?

Thanks for any ideas.

Edit: Of course when the windows are down and air is coming from the vents without the fan on the truck is in motion.

 

It must be a unique problem. lol