I see you don't have any replies yet, so I'll see if I can get things started.
Did the problem come about suddenly after some event?
The hissing sound & rough running engine suggest an air/vacuum leak. Seeing as how it seems to be coming from the A/C vent, inspect vacuum line plumbing going to the in cabin heater/ac control area. Look for loose, cracked vacuum lines to the vacuum motors that move things like the blend door. The vacuum motor diaphragm also probably belongs on the suspect list.
If nothing obvious stands out & your having problems pinpointing the hiss noise location, try a Swisher Sweets smoke test, by blowing cigar smoke into the under hood vacuum line that feeds the in cabin heater/ac controls, to see if you can locate the leak by watching where the smoke escapes.
If you do have a vacuum leak that's bad enough to cause the engine to run rough, you may have a CEL lit, so scan the computer for trouble code clues & post up All code Numbers.
Some beginning thoughts for consideration, let us know what you find.