2003 Yukon no air flow from dash vents?

I have been hearing a clutching type noise coming from under the drivers side of the dashboard for several years now. I don't hear that sound anymore. That may have something to do with the problem I'm having now. I can only imagine that there may be some sort of guide that directs the air flow to the defroster, front vents, and lower vents to the floor.

I had this problem too. its a vacumn actuator. There are many self help programs on the web that can help you. I fixed mine because it was just stuck in the closed position.