Air conditioning, vent, defrost 95 Blazer

1995 Chevy Blazer 2 dr 4.3 L 135000 mile

Temperature control and fan speed appear to be OK. Moving the control for AC, vent, heat, or defrost doesn't seem to make a difference. Most of the air seems to be coming out at the defrost and heat openings, with very little at the dash vents. The hissing sound which used to be heard as the air direction changed is now mostly constant. It seems to get a little quieter on acceleration or if the selector is in the defrost position, but other wise is there while the car is running even if set to off. What should I check to fix this?