Air/ heat problem

Recently when i go to turn on the heat or the AC the air doesnt come out of the right vent. If i set it on heat the air only comes out of the defrost and not the vents for some reason. I don't know if this has anything to do with it but when the car is on you can hear a hissing noise coming from behind the dash. Sometimes that goes away after the car is on for a while but i dont know if this is the cause. I have had resistor plug issues in hte past and plan on replacing it. I think thats the problem but can someone explain the hissing i'm hearing?

Your car's controls are vacuum operated. The hissing you hear is one of the little vacuum hoses broken or disconnected and would explain your malfunctioning vent controls.

My car made a little hissing sound as well, turned out to be the little vaccume control switch was broken, a quick easy replacement and it all works fine now, heres what the part looks like, i'd check that first. its on the back of the ac controls and you will see all the little rubber lines connected to it, this is what controls the air flow pattern: http://info.rockauto.com/getimage/getimage.php?imageurl=http%3A//info.rockauto.com/FourSeasons/36694.jpg