Blower cage might be not pressed firmly onto the motor and is grinding and slipping and not moving any air...GM blowers are notoriously noisy and unbalanced, so it could be find and the cores are crapped up with mold and dust also...so no air's getting through.
My '97's blower rumbles real loud at "3" and sounds/feels like it's gonna tear itself from it's mounts at "4"
always has. But it blows fine.
http://www.autozone.com/servlet/UiBr...3d800ed626.jsp
You'll just have to get inside the plenum and see what's going on. You have my sympathy, it sucks. I don't like doing anything under the dash...