I seldom use my air. My son was trying to use it and had the recirculation button depressed. (The air did not work. That is a different problem). The green recirculation light/button was on. After realizing the air was not going to work, I depressed the recirculation button to deactivate it to open up for fresh air again. At this point, we heard a rhythmic tapping noise from inside the dash on the passenger side. Turning the recirc button back "on" stops the tapping. I pulled the glove box to look behind. With the recirc button "unpressed", the vent door is opening, and at it's extreme open position, it keeps trying to open further, with the (screw drive?) continuously running (spinning?) creating the rhythmic snapping. Any ideas? A sensor? This should be fun to try to get at it.