Why is my aircon noisy only when it first starts up?
A noise that clears once the unit is up to speed still tells you something. Startup is the hardest moment mechanically, so it exposes play that steady running hides. How the noise began narrows the cause.
By Team Snowflake | Updated 19 Aug 2026
1. Outdoor unit rocking on its base
A dull thud or knock from outside at the moment of startup, settling once running speed steadies, usually means the unit is not sitting evenly. Concrete ledges settle over decades, and mounting bolts corrode and seize. The harder mechanical load at startup is enough to rock the unit against the mismatch.
How to tell
This path is the whole unit moving. Unlike either fan fault, the noise comes from outside and you can watch the casing shift as it starts. Unlike a worn bearing, it is a low thud rather than a metallic scrape.
- A dull thud or knock rather than a metallic rattle.
- It comes from outside, not the indoor unit.
- You can see the unit shift as it kicks in.
- It settles once the unit has been running a moment.
How we confirm it
We check all four mounting points for even contact and corrosion. Anything not bearing load is packed or replaced, then the unit is restarted to confirm it no longer shifts.
Never leave an outdoor unit that visibly rocks at startup. A mounting point that has already let go puts the remaining fixings under load they were not sized for, and the noise is the warning.
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2. A loose fan part wobbling at low speed
A rattle that appeared fairly suddenly, clears within a moment of spin-up, and comes from the indoor unit usually means a fan part has drifted on its shaft. A barrel held by a single screw can work loose over years of vibration. Off-centre parts wobble hardest at low speed, so the noise lands at startup.
How to tell
This path is a discrete part that has moved. Unlike a worn bearing, it appeared suddenly rather than creeping in over months, and the sound is a loose knock rather than a grind. Unlike a rocking base, it comes from indoors.
- The rattle started suddenly rather than creeping in.
- It comes from the indoor unit.
- It clears within a moment of the fan getting up to speed.
- The sound is a loose knock or rattle, not a grinding.
How we confirm it
We open the indoor unit and check the fan barrel position and its fixing on the shaft. It is reseated centrally and secured, then run from cold to confirm the spin-up is clean.
Do not keep running a fan barrel that has drifted on its shaft. Its edge already sits close to the housing, and contact at full speed will break it.
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3. Worn bearing letting the rotor wobble
A metallic scrape or whirr at startup that has crept in gradually over months, on an older unit, points to bearing wear. Bearings hold the shaft at a tight clearance. As they wear, the shaft gains side play and brushes its housing while accelerating from rest, then centres itself once up to speed.
How to tell
This path is progressive wear rather than something that moved. Unlike a loose fan part, it crept in over months and grinds instead of rattling. Unlike a rocking base, it comes from indoors and worsens slowly.
- It crept in over months rather than starting suddenly.
- The sound is a metallic scrape or whirr, not a loose rattle.
- The unit is old, and the noise is slowly getting longer.
- Airflow may have dropped slightly alongside the noise.
How we confirm it
We check shaft play by hand with the unit isolated, and confirm the wear against airflow and current readings. The motor is then quoted against the age and condition of the rest of the unit.
Lubricating a worn sealed bearing does not fix it. The quiet lasts a short while and the noise comes back, so paying for that twice is worse than pricing the motor once.
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