September 8, 2026 · Cairns Aircon Installers
A modern split system should be close to silent indoors and no louder than a quiet conversation outside. When that changes, the sound itself tells you a lot. Different faults make distinctly different noises, and knowing which one you have is most of the diagnosis.
Rattling Or Vibrating
Usually the least serious, and often the easiest to fix. Something has worked loose with vibration over time.
Common sources are the outdoor unit's casing panels and screws, the mounting feet or brackets, a fan guard, or debris that has found its way inside the outdoor unit. In Cairns we pull an impressive amount of leaf litter, seed pods and the occasional gecko out of condensers.
If the rattle changes or stops when you press gently on a panel, you have found it. If it comes from deep inside the unit, leave it to a technician rather than removing covers.
Buzzing Or Loud Humming
Electrical, and worth attention. A hum from the outdoor unit with the fan not turning usually points to a failing capacitor or a motor that cannot start. A buzz from the isolator or from inside the casing can indicate a loose connection, and loose connections generate heat.
If the system also trips the switchboard, read our post on why air conditioners trip the breaker, because those two symptoms together narrow it down considerably.
Clicking
A click or two when the system starts and stops is completely normal. That is relays and contactors doing their job, and the plastic housing expanding and contracting as it changes temperature.
Continuous clicking is not normal. Repeated clicking without the system starting usually means a control fault or a component trying and failing to engage, and it is worth a look before something burns out.
Screeching, Squealing Or Grinding
Take this one seriously. High-pitched metal noise means bearings.
A fan motor bearing that has dried out or corroded will squeal first, then grind, then seize. Once it seizes you are replacing the motor rather than servicing it, and a seized motor drawing locked-rotor current can take other components with it. Salt air on the coastal suburbs is hard on bearings, so Palm Cove, Trinity Beach, Yorkeys Knob and Holloways are over-represented in these calls.
A screech from the compressor itself can indicate internal pressure problems. Either way, switch it off and book it in.
Gurgling Or Bubbling
Two possibilities, and they are easy to tell apart.
Gurgling from the indoor unit or from the drain line is usually the condensate drain, either partly blocked or drawing air. It is often the early warning that comes before the unit starts leaking water through your ceiling, so it is a good one to act on rather than ignore.
A bubbling or hissing in the refrigerant pipework can indicate a refrigerant leak with vapour and liquid mixing where they should not be. That needs a licensed technician.
Banging Or Clanking
A hard bang from the outdoor unit is the one that worries us most. It can mean compressor mounts have failed and internal components are moving, or that something has come loose inside a rotating assembly. Running it after that point risks turning a component failure into a whole-unit failure.
Switch it off at the isolator and get it looked at before running it again.
Hissing
A gentle hiss for a few seconds after shutdown is normal pressure equalising. A persistent hiss while running is not, and refrigerant escaping under pressure is the usual explanation. Because these are sealed systems, a low charge always means a leak. Topping it up without finding the leak is not a repair.
Water Hammer Or Knocking In The Walls
Occasionally what sounds like an aircon fault is pipework expanding against a joist or a bracket as it changes temperature. It ticks or knocks on startup and shutdown and does nothing in between. Harmless, though a technician can usually pack or re-clip the offending section and quieten it down.
What Makes A System Get Noisier
Age is part of it, but installation quality matters more than most people expect. Outdoor units on undersized or poorly fixed brackets transmit vibration into the building. Units mounted hard against a wall with no clearance resonate. Pipework clipped too tightly telegraphs every expansion. Where the unit sits and how it is fixed is worth getting right the first time, which is why we wrote a separate guide on where a split system should be installed.
Regular servicing helps too. Bearings that stay clean and clear last longer, and loose fixings get picked up before they become a rattle you live with for two years.
If Yours Has Changed
A new noise is a change of state, and changes of state in machinery are worth investigating early. Have a look at our repairs and servicing pages, or get in touch and describe the noise. Honestly, it helps enormously if you can record it on your phone.