ok, the problem is assembly adhesive from the ducting getting down into the seal area for the air door. The door that makes the bang noise is right behind the two swivel vents above the radio/AC controler in the center of the dash. If you shine a light through the vents and work the controller you will see it moving up and down.
The problem with the glue is that it gets hot and flows down under the air door seal, when it cools down at night or something it hardens and glues the air door shut.
If you just have a minor sticking: get some silicone spray, some have recomended WD-40 but I prefer silicone spray because it doesn't smell. anyhow, get a spray tube for the silicone that you can fit down through the swivel vents. slide your AC controller over to vent to open the door and hold the door up with a thin screwdriver etc. and then turn the air off. Spray the silicone spray around the edge of the door where it seals and down on the duct where the door rests when closed. This will cure most minor sticking.
If you have a really hard sticking door (mine sounded like someone was shooting a .22 under the hood): You will need to take the face plate that runs across the passanger side of the dash off, the one with the three sets of vents and the MC badge on the far right. This face plate is a snap in affair except the two top fasteners which are a little bent over metal clip that comes through the dash. To remove the metal clips, take the screw out of the limit cable on the glove box, this will allow you to pivot the glove box all the way down towards the floor. You can now reach up behind the dash and pull the clips out or bend the wings in so you can pull the face plate off. With the face plate off, remove the duct that is behind where the two center swivel vents were, there is one screw on either side. Now you need to get the door in the up position, I had to pry mine loose. you can either jam something in there or attach a mity-vac to the blue line in the vaccume harnes going to the AC controller that you will see behind the glove box. you should be able to see a bunch of glue where the air door seals in the down position, scrape it out with a gasket scraper and clean everything off, then spray silicone on the door seal and the duct where you scraped the glue. assemble in reverse order.
I would also recomend that you replace the hose that runs this whole mess. Start at the round ball shaped plastic vaccume tank in the drivers side fender behind the headlights. Replace the long hose from the ball back to the distributor area where it T's, replace the short piece that goes to the cruise, and the piece to the engine. The line to the engine will hook up to a round plastic piece, it is a one way valve and needs to go back in the same direction. The other piece will be a plastic line that goes around the back of the engine and connects to the white/gray line that goes inside the car. I like to replace the plastic line with rubber from the T over to the connection in the heater hose area. You will need to cut one end of the black plastic line to get the connector. If you just start at the plastic ball and trace the line back and around the engine this will make much more sense, I took me about 15 minutes to replace all of the lines.