Help with Air Conditioner...

The air conditioner just started making a hissing noise when it is turned off. It seems to still be working ok. Any thoughts on what it could be?

If the system uses a fixed orfice; it is just the high side pressure moving throught the orfice to the low side. A/C systems have a high pressure side and low pressure side. when you turn the system off the pressure from high side equallizes to the low side. It moves throught a calibrated restriction called a fixed orfice.

this occurs anytime the a/c is turned off ...you just can't hear it with the car running.

It stopped working all together now. Is this a switching problem and if so, how can you fix this without too much trouble?

Just check if the A/C is on but placed in low pressure volume. it makes hissing sound. but not to high sound. or take the a/c to a mechanic and check with him.