2000 S80 Air Conditioner not cooling, hissing sound

[cgsnipe]

2000 S80 2000,000 miles, Air conditioner does not cool, compressor is engaged when called for and does not cycle. I began hearing a hissing noise, like the refrigerant moving through the evaporator in the dash, and it has gotten progressively louder. I have not taken it in to an expensive service facility yet nor had a manifold set hooked up to determine the pressures. Just curious why the compressor clutch is constantly engaged, but no cooling. Heater/cooling doors are functioning normal. Thoughts?