What causes the hissing noise in an A/C system after turning off the vehicle?

The hissing noise that occurs in an A/C system after the vehicle is turned off can indeed be attributed to a combination of normal conditions and the equalization of system pressures.

When the A/C system operates, it maintains a certain level of pressure to facilitate the refrigeration cycle. After the system is turned off, the refrigerant is no longer being actively cycled, allowing the high-pressure side and low-pressure side of the system to gradually equalize. This equalization process can generate a hissing sound, as refrigerant moves through the expansion valve and into the evaporator.

Additionally, it is completely normal for some refrigerant to expand and cause a hissing noise as it transitions between states when the system is depressurized. This natural behavior of the refrigerant upon shutdown is not indicative of a problem, provided that the noise is not accompanied by other signs of a refrigerant leak or malfunction.

Therefore, recognizing that the hissing noise can result from both the normal functioning of the system and the equalization of pressures helps understand the A/C system's behavior after operating conditions change.