what is causing hissing sound when AC is turned on?

Hello, Since it only occurs while the A/C is running, I would suspect you are hearing the expansion valve opening and closing to spray coolant into the evaporator core. This may be louder than normal because the evaporator core or expansion valve may be slightly different that the OEM parts or there could be some of the factory insulation material missing. Also, if the air conditioner was not working for a long time, you may have just forgotten what it sounds like, though you would be the best judge of that. If you notice the A/C is not getting cold, or gets less cold, you may have a small leak when the expansion valve is fully open, though I would not anticipate this reaction from the system.

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