Why is my aircon hissing while running?
A hiss while the aircon runs is alarming because it sounds like escaping gas. Three very different faults share that sound: harmless pressure equalising, a slow refrigerant leak, and electrical arcing. The timing, cooling trend, and smell tell them apart.
By Team Snowflake | Reviewed 30 May 2026
1. Normal short expansion noise
You hear a brief hiss or whoosh right as the compressor starts or stops, then silence. As the compressor changes state, refrigerant pressure equalises across the system and moves through the expansion valve, which makes that short sound. Cooling stays steady and the noise repeats predictably at each cycle transition.
How to tell
Normal expansion hissing is tied to startup or shutdown, not steady mid-cycle running. Unlike a refrigerant leak, it stops once pressure equalises and cooling never weakens. Unlike electrical arcing, it carries no smell and never trips the breaker. The absence of cooling loss is the clearest sign.
- Hiss is brief and lines up with the unit starting or stopping.
- Cooling stays consistent through the cycle.
- No warning lights, no smell, no breaker trips.
How we confirm it
We listen at the expansion valve area during cycle transitions. Then we compare how long the sound lasts against normal behaviour for that model. Only then do we treat it as a fault.
Do not let a normal startup hiss trigger a gas top-up or part swap. Confirm the timing window first, or the work adds scope the unit never needed.
2. Refrigerant leak or restriction pattern
The hiss is faint but continuous while the unit runs, and the room cools more slowly than it used to. A small opening at a flare fitting, brazed joint, or valve packing lets pressurised refrigerant escape, making turbulent gas noise. As the charge drops, the coil runs colder, and cooling fades over weeks.
How to tell
A refrigerant-leak hiss runs through steady mid-cycle operation, not just at transitions. Cooling deteriorates over time: the room takes longer to reach setpoint, then stops reaching it. Unlike normal expansion noise, it only worsens. Unlike electrical arcing, it produces no smell, never trips the breaker, and stays smooth and low rather than sharp.
- Faint hiss continues through steady running, not just at startup.
- Room takes longer to cool than it used to.
- Cooling quality drops further week after week.
How we confirm it
We pressure-test the refrigerant circuit and inspect flare connections, brazed joints, and the service valve for leak signatures before any gas work is recommended.
Do not accept a gas top-up before a leak check. Topping up without finding the leak repeats the same complaint after the charge bleeds out again.
3. Electrical arcing or stress noise
The sound is sharp and irregular, more like snapping or crackling than a smooth hiss, and it may come with a burning smell. A loose terminal or degraded connection lets current jump the gap and break repeatedly, each break making the crack. This starts most often at the outdoor terminal block, where compressor vibration works connections loose.
How to tell
Electrical arcing is harsh and irregular, snapping rather than smooth. Unlike a refrigerant hiss, the giveaway is signs unrelated to gas: a burning smell from charred insulation, a breaker that trips, or heat discolouration near terminals. Unlike expansion noise, the sound changes character with each restart instead of staying predictable.
- Sound is harsh and irregular, like snapping or crackling.
- A burning or acrid smell appears near the unit.
- Breaker trips on startup or mid-cycle, or terminals look heat-darkened.
How we confirm it
Stop restarting the unit. We run a full electrical and control safety check covering the terminal block, contactor, and wiring insulation before any restart.
Stop using the unit if you smell burning. Turn the breaker off and do not attempt a restart until the wiring fault has been confirmed and repaired.
Related reading
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