Whenever I turn on the A/C or Defroster (runs the A/C compressor as well) I hear a loud hissing coming from the passenger side of the dash. Much louder than the noise of the vacuum controls of the HVAC. Sounds like a HUGE vacuum leak. But the idle doesn't rise or fall, so it shouldn't be a vacuum leak of that magnitude?

Does this mean that the evaporator core is leaking??

 

i do commercial a/c and ive done my own. get one of those autozone a/c gauges and check the pressure. its cold out so it can seem low. but it will help. the a/c comes on with everything but cabin vents. come on with feet,windsheild, and mix. then of course a/c and high a/c. if you have a vac leak it right by the floor pass side is the blend door with an vac acuator.

well maybe not cold for you didnt see you were in ca