hissing noise under dash when HVAC on
 

had a very annoying hissing noise under my dash for years. sounded like Darth Vader lived there. everything worked so we left it alone. truth is I could never find any suggestion as what the problem could be, as everything was working, don't fix what isn't broke. only happened when the hvac system was on, heat or ac. last week it stopped, and so did the ac. Heat works, all vents work as selected, the compressor does not engage when turned on. No error codes, it is 2005 with 150k. original owner, been a pretty good car.

U did not read the posts did you?

The air conditioning system requires making use of the refrigeration cycle: Refrigerant as a gas is compressed by the compressor, which results in heating it up. The hot gas is cooled in the condensor which results in it turning to a liquid, still under high pressure. The high pressure liquid expands suddenly at the expansion valve into the evaporator core located under the dash where, because of the sudden expansion and drop in pressure, it becomes cool and where it can absorb the heat in the interior of the car.
That hissing noise you heard was the expansion of the refrigerant. AC not working, noise stopped.
After 9 years and 150,000 miles, it's not surprising that the AC system needs service.