Update:
After I posted that it hisses in anything but "OFF," I came out this morning to do a reality check on that since I rarely use anything other than AC and Defrost. It turns out that it only hisses in those two modes.
I did try two methods for retrieving codes. "OFF+Panel" then "AUTO" It went through the count then all sections illuminated. "OFF+FLOOR" then "AUTO" or "DEFROST" (can't remember exact sequence but I had my note sheet in front of me when I did it), all sections illuminated. I heard the vents cycling each time I performed either one of these functions. There did seem to be a slight clicking sound at some point doing the cycle, but I don't know if that's normal or not.
BTW, the hissing is fairly slow while idling or waiting at a stop light, and speeds up and gets louder as I accelerate.
The sound is similar to when you first switch to "DEFROST" on the climate controller in any other car. You here a short hiss then it sounds like a "flap" closes and hissing goes away. In my case, the hissing doesn't go away. Are there any vacuum components to go bad on the 02?
Thanks again