Just got this car and am still figuring things out. The hissing starts up when i hit the off button for the fan, and it sound like its right behind the ac/heat console.

Does it make that sound for just a few seconds...?

 

Mine does the same thing (86 GL sedan) and my 92 Loyale & 85 DL did the same thing... I kinda figured it was the sound of the system turning off and expelling pressurized air...

 

So unless you think you have a boa in your dashboard, I think its nothing to worry about, at least in my experience of 3 Subarus that have done that...

I am not 100% sure but I think that the 'blend door' is vaccum accutated and thats what the sound could be. My 88GL does the same and I don't have any noticeable issue there.

 

Unless...it IS a boa...?

Look behind the driver side of the center console under the dash. There is a rubber fitting that contains nylon vac lines form the firewall, and it plugs into the door actuator next to the heat adjust cable. See if it has been knocked loose. You may have to drop the kick panel to get a good look at it.

Every subaru I've had has done this...Is there a little loss of power you can feel that goes along with it?

If the hiss is momentary it's fine. Just the vacuum in the controls system bleeding off when you turn it off, or sometimes when changing controls from Vent to Defrost to Heat etc....

 

If you hear a constant, continual hiss, one of the lines is loose. Look at the junction Miles describes.