You post yesterday got me started on this and I figured it out. Its not coming from the dash at all. Everyone in cluding me was wrong. Its coming form the overhead lights. Open up the over head lights per owner's manual instructions. In there you'll find a small temperature probe with 2 wires and a vacuum hose going to it. The vacuum hose is there to constantly bring fresh air from the cabin around the temp sensor for it to get an accurate reading. The temp sensor sends a signal to the electronic climate control to maintain constant temp in the cabin. There is a second temp sensor in the middle of the speaker on the passenger side of the dash. Over time the little orafice around the sensor gets clogged with dust particles which causes it to hiss. Clean off the dush and no more hissing.
Good luck
Justin