Heat / AC Flapper

When I have the selector set to bring air in from the outside I can hear the flapper in the dash trying to move, it will sound like it smacks something then over and over. What could be the problem?

When I have the selector set to bring air in from the outside I can hear the flapper in the dash trying to move, it will sound like it smacks something then over and over. What could be the problem?
What year? I have a '98 Montana and I am well versed in this area. There is a little motor that controls that door. It can be removed from the vehicle and be dis-assembled. What it will show inside it is a broken gear. I went to a junk-yard and picked one up by looking under the dashes of numerous vehicles until I found one. New, it will run you about $200 or more. It is located under the dash at the very top on the passenger side. You remove the trim on the side pillars, then pop up the dash portion that is near the windshield and you can see it. You also have to remove some plastic under the dash on the passenger side so that you can simultaneously reach your left hand up from underneath and your right hand down from the top to get it out. I think it is held in by two phillips screws and has an electrical connection to it.
http://i486.photobucket.com/albums/rr230/rkvons/GMBlendActuator.jpg

I should have mentioned the year in my original post. Sorry. My Montana is a 2002 model.

Do you know if the 1998 and 2002 Montana have the same mechanism?