Okay I got a good one.....I noticed today a hissing sound coming from under my dash when stopped at a light (radio off). It sounds like a vacuum leak then I realized that the ac is blowing out warm air. After an hour of checking things out, I realized the sound happens when the ac button is "on", whether in auto mode or pressing it manually. Turn it off and hissing sound fades away. The ac compressor is running fine and the high side of the ac hoses are cold. The problem seems to be something in the HVAC unit is not switching from hot to cold when there is a demand for ac. Is there a vacuum actuator that opens and closes any door in the HVAC unit? There is usually a door that swings to divert air either through the evaporator unit or heater core depending on the command for heat or ac. Any other idea's? Is this a common problem?

 

Could be the intake door control or air mix system. There's like four different sensors that input signals to the control system. No vacuum lines that I'm aware of.

 

hissing sound and warm air sounds like the refrigerant is leaking out of the system.

 

Been having a similar issue in my 04 for awhile. Only mine happens when i first turn the air on and lasts a few seconds. Its a slow leak so it would take about 6 or so months for the freon to leak out lol. Was told it was the expansion valve and dash will have to be removed to replace it.

Good luck!

 

definitely sound like your hose is crack and leaking or evap core is cracked. who knows

 

Update.....turned out to be a defective a/c condenser. The dealer replaced it free under warranty. I found it odd though that the compressor was still running with low/no refrigerant. Whatever the case, it made a funny hissing noise from under the dash when the a/c was on. Its been a week now and the car is still 100%.