2002 Chevy Cavalier Cabin air circulation system

Air Conditioning problem
2002 Chevy Cavalier 4 cyl Two Wheel Drive Automatic

About a month or more ago I noticed a strange sound coming from behind the dash while switching between different air settings. Unfortunately, I don't remember which settings they were, but the sound can be described as something struggling to move between one of two positions. As of now, this sound no longer occurs.
The result is, when I have the control set to blow air out of the top vents only, I get about 40% out of the top vents and the remaining 60% out of the bottom vents.
I purchased a Haynes repair manual, but it has done very little in helping me diagnose the problem.
I would love to get someones input before I attempt to explore beneath the dash.
Thank you.