Yet another strange noise...

I did a little trimming on the front fenders and I am making a new front bumper (kinda). In the process of removing the front bumper and trimming the fenders, I discovered the vacuum canister behind the bumper on the pass. side. I pulled it off to relocate it and broke the vacuum tee in the process. I didn't have another one, so I fired it up and it ran ok, so I figured I would get it later. Well, it didn't take too long to discover that it was for the HVAC controls because I was stuck with the heater on in 70 degree weather. I have relocated the canister to right in front if the coolant bottle and got it all hooked back up but I now get a hissing sound under the dash on the pass side every few minutes. I assume it has to do with the HVAC but why is it doing it?

You wrote that you broke the tee fitting. If your repair isn't 100% vacuum-tight, perhaps the system pulls a vacuum and then releases it (leaks), and what you're hearing is the vacuum dampers in the dash shifting position as they lose pressure.

Strictly a guess. But the system uses vacuum to control the air flow as well as the temperature. When it loses vacuum, it defaults to directing air flow through the defroster outlets on the top of the dash, regardless of where the controls are set. Check where the air is flowing when you hear the hiss.

That's good info Eagle. I replaced the tee, but I may have a leak somewhere else. I will check it out. Thanks.