gotta be a vacuum leak.
It's dark out there so I can't compare to mine, but someone here knows.
Doesn't sound like a compressor problem, just a vacuum fitting problem (which is good compared to a compressor!)
When the car is running does the compressor kick on? Do the light's dim a little? How about your vents? When you hear the hissing, and change vent settings do they move, or move really slowly?
