Ford F150 - 95 AC/Vent Air flow obstuction

95 F150 4x4. The air flow for the AC and Vent is obstructed until a "thump" in the center of the instrument panel occurs. The fan is on and the vacuum valve under the hood on the passenger side is operating properly. The problem started with the AC but now occurs when the Vent is used. The "thump" sounds like a damper closing or opening. Any ideas where the damper is or what is causing the problem?

Thanks.

It's the panel diverter door in the L end of the HVAC box. They're known to break off their hinges, but observe the vacuum motor's operation while changing the settings with the engine running. It's the one at the bottom center of the photo:

.