Well I am guessing it is a vacuum leak of some sort. The place that the air is set to come out doesn't always work, so I believe it that to be the source. The air is always blowing out of the defrosters and sometimes eventually gets rerouted to the set output(vents, floor). Would this be an internal leak in the control unit, or just a hole in the hose behind the unit that I could easily replace or is there not enough info to tell and I should just get of my butt and take it apart and find out for myself?

Chris

 

Most HVAC controls will make a slight hissing noise. Well, at least my last 2 did. If its loud enough where it sounds like a leak then yes your gonna have to take it apart and see where its coming from back there.

 

The connection from the vac lines to the back of the function switch is leaking. When it does, it hisses and blows out the defroster vents. It takes vacuum to run any other function. You'll have to pull the sliding switch assembly loose so you can see the back of it. Once there, you'll see a bundle of vacuum lines plugged into the switch using a large common connector. This is more than likely where your problem is. The connector is held on with two push nuts if I remember right. You can remove them, unplug the connector, and then put it all back together using your choice of sealant. Be careful not to plug any of the lines or nipples if you go overboard with the sealant though.