Noise When Engine Turned Off

When the engine is turned "OFF" with the A/C system operating, the TXV closes completely and high & low-side pressures take a long time to equalize (they will eventually equalize due to a very small amount of bleed back through the compressor reeds). A properly operating TXV A/C system should not "Hiss" like an orifice (expansion) tube system does when shut down.