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Hello my name is***** am an ASE Master Technician and I am glad to help you out with your question.
Does the hissing sound happen when the rear blower is on?
You're hearing refrigerant flow through the orifice at the rear evaporator core. It echoes through the overhead duct work, and is rythmic with the cycling of the compressor. You'll hear the same "hissing" noise from every Expy/Navi with rear A/C. It's more pronounced when the vehicle is a bit low on refrigerant.
You could have a slight leak and the refrigerant is slightly low, this could cause a hissing sound. I would say evac and recharge the system adding leak detection dye and drive it for a while, then use a uv light to find the leak.
Replace the leaking part and recharge, and the hissing noise will go away.
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ASE Master Technician
Hello again Dan here,
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