In regards to the loud hiss it's most likely a hiss in your Freon system. Does the air blow cold on a hot day? I know, crappy time of year to test it. I would figure you probably have a leak in your A/C system some where. Unfortunately, you'll have to get it checked by a pro.
As for the water sounds in the Vents it's most probable that your system is "LOW" on gas; your A/C system that is. Not a big deal just take it in and get it charged up. My 90' Blazer was doing that before I sold it. Wasn't a biggy as it ran fine and everything. Probably just a little on the low side for freon.
Hope this helps
