heat/vent control hissing

The heat only comes out the defrost vent and not the others and I can hear a hissing sound coming from the back of the vent control. Sounds like a vacuum line. How can I fix?

Hey-- I thought I was the only one with that problem!!!!! But I found out im not-- thank you. Im getting ready to have a friend fix it-- he knows what it is-- Ill find out from him and let you know.

I have the same problem. Mine is comming out of the switch on the dash where the vacuum lines come together. They are very loose by now so I think I have to find out a way to seal them in(silicone???)

Wow how frustrating to have this problem. Did any one have an answer as to what this noise is? AC does not work and it's 90 degrees lately. Is this something that I could tackle myself or should I hang it up and take it to my mechanic again. He tells me to get a new vehicle (just not practicle right now if you know what I mean).