Blend door slams shut

[project nightmare]

I have a 96 Tahoe, and sometimes while I'm driving it sounds like the blend door slams shut. I think thats what it is anyways. Has anybody else experienced this? Or does anybody have an explanation on why it might do this?

 

[SunlitComet]

So this happens with no input on the dial? I assume we are talking about the temperature control door.