It's not a slight hiss when the refriderant is low. It's rather noticable, and sounds like it's coming from right behind the center vents.

If it's low, you won't damage anything. If the expansion valve is faulty, you probably won't damage anything, but may dump refridgerant if the head pressure gets too high.

What's #7 say on N22 in sensor readout mode when you run the AC?