My 1994 yokun makes a hissing noise for about 10-15 seconds when a/c is shutting on 1994 GMC Yukon

down......I had a new compressor put in by a mechanic. A/C then worked on and off......replaced the condensor again he then said my a/c filter needed to be replaced. Which he did. Now, when I turn the truck off (after the a/c unit has been running), it makes a hissing noise coming from the passenger side for 10-15 seconds. They said that's normal when the a/c is shutting down. I think they are taking me for a ride!!! Please help......
Any suggestions or what I should say to them......

Asked by for the 1994 GMC Yukon
It is normal. It is the sound of pressures equalizing at the orifice. If you put a compressor/condensor to try and fix the noise it was a waste of money. You did not say but did you just buy the vehicle? That was a common noise for GM vehicles and if you never owned a GM then maybe you didn't know this. It is always important to tell us everything when asking for help-like did you just buy it!
I have owned this vehicle since 1994. The compressor/condensor was replaced as they said I had a leak. The noise only appeared after they replaced the condensor/compressor and the a/c filter. It has NEVER made a hissing "shut down' noise when I turned the engine off after running the A/C. And I don't hear that "shut down noise" in other cars either. The loud hissing noise just became after the repair shop replaced the A/C condensor and the "A/C/ filter assembly". The A/C works, just that there is a loud release of pressure that I have never ever heard mine, or any other car. Any you think a hose or relief valve or something was broke when the re-einstalling the new parts?
Very difficult to say without having the vehicle in front of me. I personally have worked on and owned GM vehicles that did this noise and all was working perfectly. If it hasn't been done then have a shop replace the orifice, where the noise is happening, and see what happens. Typically though they are changed by the shop when any compressor is changed. You had not said whether it was replaced. It may be itemized on the compressor replacement bill.