Hissing/whining noise from HVAC

Hey, I just noticed today that when I have my vent position set on defrost it makes a hissing/whining noise that seems to be coming from the right side. It blows out plenty of air and its hot but ONLY when its set on defrost does it make this noise, when i slide it heat or vent it gradually goes away in about 5 seconds. any ideas? btw its a 96 4.0/auto with just about 170,000 on it, thanks in advance

hissing noise would signal a vacuum leak to me.

yeah thats what came to my mind, but I didn't know why it would only be when its on defrost and why it isn't affecting the performance of the whole system

perhaps the leak is at the vacuum motor, so it only hisses when that motor is activated. I'm not to sure about the HVAC vacuum system. I've not gotten in to mine, it's one of the few things that works properly on the MJ.

My dodge truck does the same thing for a few even after the engine is shut off. Been like since I got it years ago and still workin bueno. Cant say on the 95 chero cause I ripped the AC out.

yeah with my two blazers and both of my jeeps if you slide the little button to "off" when its on heat or vent or where ever when the car is off it makes a little hissing noise so thats normal I would say. My other jeep didn't even have a vac. canister so it would only blow air out of the defrost vents anyhow, so I guess I won't complain about this since it still works fine