Why is my aircon noisy only at certain fan speeds?
A noise tied to one fan speed is a clue, not a coincidence. Every speed produces a different vibration and a different air velocity. Which speed sets it off narrows what is loose.
By Team Snowflake | Updated 19 Aug 2026
1. A fan part grazing at one speed
A rattle or scrape that appears at one middle speed, and clears both above and below it, usually means a fan part sits slightly off-centre. A retaining clip works loose over years of vibration. The wobble is widest at one particular speed, so the blade tip grazes its housing only there.
How to tell
This path needs one specific speed. Unlike a duct whistle, it appears at a middle setting rather than only at the top. Unlike a chattering cover, pressing on the casing changes nothing.
- The noise appears at one speed and clears at the others.
- It is a scrape or rattle from inside the unit.
- It came on gradually over years rather than suddenly.
- Airflow itself seems unaffected.
How we confirm it
We open the unit and check the fan blade seating and its retaining clip. The blade is recentred and secured, then run through every speed to confirm the graze is gone.
Do not keep running a blade that grazes its housing. Each contact wears the tip and worsens the imbalance, which turns a clip adjustment into a fan replacement.
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2. Air whistling through a loose duct or gap
A whistle or rush that arrives only at the highest speed points to a gap in the air path rather than a moving part. A clamp left loose after a layout change leaves a small opening. At low speed the air passes it quietly. At full speed the velocity through that gap is enough to make it sing.
How to tell
This path is air, not metal. Unlike a grazing fan part, it only appears at the very top speed and sounds like rushing rather than knocking. Unlike a chattering cover, it comes from the duct run rather than the casing surface.
- Only the highest fan speed brings it on.
- It is a whistle or rush of air, not a rattle.
- The ductwork or ceiling was worked on at some point.
- The noise is steady rather than intermittent.
How we confirm it
We trace the whistle back along the duct run to the joint that is leaking air. That clamp or connection is refastened and sealed, then tested at full speed.
Do not let anyone replace the fan motor for a whistle. The motor is turning normally, and the noise is air escaping a joint that was never refastened.
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3. A cover or panel chattering against the wall
A buzz you can stop by pressing on the casing is not coming from inside the unit. A trunking cover or access panel that has worked loose vibrates when the fan runs at the speed that matches it. It is most noticeable at night, when the room is quiet enough to hear it.
How to tell
This path can be silenced by hand. Unlike either internal fault, pressing on the cover stops the noise immediately. The source is a surface panel rather than anything spinning or any air gap.
- Pressing a hand on the cover or panel stops the noise.
- It sounds like it comes from the wall, not the unit.
- Most noticeable at night in a quiet room.
- The sound changes character as the speed changes.
How we confirm it
We find which cover or panel is resonating and refasten or pack it so it sits tight. The unit then runs through every speed to confirm nothing chatters.
A fan or motor quote for this misses a loose cover. If a hand on the panel silences it, nothing inside the unit is at fault and no part needs replacing.
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