Pontiac Repair: Hissing-Driver side of Engine-Pontiac Grand Prix 3.1 V6, pontiac grand prix, power steering pump

Question
I first noticed a hissing sound coming through my inside a/c vents.   I hear it while driving when I have the a/c turned off and it persists from 10-30 seconds, depending on how long I've been driving, after I turn the engine off.  The origin of the hissing appears to be the driver side of the engine toward the top to top-middle.  I do not see any fluids coming out anywhere.  Any ideas of what this might be and what I need to do to fix it?

Answer
Hi David:

Noises are hard to pin pint especially if I can't hera it. But I have 2 thoughts wither a power steering pump is making noise or it's in the a/c system it's self. Does the a/c cool the car off well or is it kind of cool not cold? You could be hearing the refrigerant going through the evaporator which is the cooil coil inside the car. if the ssystem is low on a chage it will #1 not cool correctly and #2 make some wierd noises from time to time. And the fact that the noise is there when you turn the car off rules out am kind of power steering issue. but if the a/c was on when you turned the car off as the pressures equalize in the a/c lines that could be the root of the noise you hear. Do you know if this car still has the origianl r-12 gas in it or it was retrofitted with the new 134A gas? If it was retro fitted and it was charged incorretly then that would make scence as well. Good luck :)