1991 Chevy Silverado

There are electronic actuators that open and close doors in your evaporator case. It sounds like a door is in a bind and as the actuator tries to open or close it, it starts bumping against whatever it's stuck on. I've heard the noise before, several times. The actuator could be at fault, as well. Depending on which one it is, (I think there's 3) you can remove it, plug it in and activate your controls and see if it still makes the noise when it's not actually moving the door. If it does, replace the actuator. Either way, that's what's happening. Some actuators can be replaced in the vehicle. Others will require the evap. Case to be removed from the vehicle. Good luck.

You're more than welcome. I'm glad I could help. Feel free to ask again if further problems arise!