What could be causing a hissing sound from an air conditioner?

Answer by  TheRealPlumber (249)

The hissing sound that you hear is caused by the high pressure suction return line from your A/C unit. Although this is common you might just need to consider having your system looked at by a Pro, you could need a recharge of gas.

Answer by  kipcat (1964)

The air conditioner, when it shuts down, will make a hissing sound when the unit equalizes. The unit can also have a hissing sound when the outdoor air temperature is 95 or greater because this causes a higher pressure differential than you normally hear during 70-80% of the operating time.