Blower fan shuts a door when on high.

You may have a broken door

When on high, what you're probably hearing is a door opening. At Max, all the blend doors should be open. On a twenty year old Pontiac, I'd be surprised if the doors are functioning properly at all.

Most likely a broken door. The defrost door is a common one to break on GMs. Hope that helps! Jim

I bought the car about week ago so don't know to much about it. Found that the blend door motor must have been replaced at one time. I found the the motor was working backwards so when you increase the temperature the door was closing. Took the motor apart and reversed the connection and it works fine