2000 Windbag heat/cold door flapping ?

When I start my 2000 Windstar the heat/cold control door is flapping. I can hear it trying to open or close. If I adjust the temp control lever, it just sounds different but it doesn't work. After a few minutes, it stops making noise. I know this is vacume controlled but my question is...do you think this is a vacume problem or a door motor problem. I don't hear any vacume leaks and don't have a tester. Should I get a tester before I pull the dash apart to get at the motor?? Thanks for any responses.

Rob
let's not meet by accident :rofl:

do you hear any clicking? If you do, then it's the actuator motor that has plastic gears. they eventually break.