Aircon Cassette Rattle, Loose Fan Blade

Case Details

What We Checked

• Rattle present at medium fan speed but absent at low and high — pattern inconsistent with compressor bearing noise.
• Compressor sound smooth at all indoor fan settings, no roughness or grinding.
• Fan blade retaining clip had shifted on the motor shaft, allowing the blade to wobble at the resonant speed.

The Diagnosis

The retaining clip on the motor shaft had loosened over five years of normal vibration, allowing the fan blade to drift slightly off-centre. At medium speed, the wobble amplitude was just enough for the blade tip to graze the plastic housing on each rotation, producing the rattle. At low speed, the rotational displacement was too small to close the gap, so there was no contact. At high speed, centrifugal force pulled the blade outward on the shaft, moving it past the contact point and into a stable orbit. This speed-dependent behaviour is the hallmark of a loose rotating component — it resonates at one RPM band and is quiet everywhere else. Compressor bearing noise, by contrast, stays constant regardless of indoor fan speed because the compressor runs independently on a separate circuit.

What Fixed It

We re-secured the retaining clip on the motor shaft, repositioned the fan blade to sit true and concentric, and confirmed there was no remaining play by spinning the blade by hand before powering on. We then ran the unit through all three fan speeds to verify the rattle was gone at every setting. No parts were replaced, no refrigerant work was done, and no work was needed on the compressor or outdoor unit. We also checked for any scoring marks on the housing where the blade had been making contact — the wear was superficial and did not require housing replacement. The entire fix took under thirty minutes on-site.

The rattle was gone on the same visit. The office avoided the outdoor unit replacement that had been quoted by the other contractor.

Why This Happens

Speed-dependent rattle — fan assembly vs compressor bearing.

• Compressor bearing noise stays constant regardless of indoor fan speed because the compressor runs on a separate circuit outdoors. A rattle that appears at one speed but vanishes at others belongs to the indoor fan assembly — the speed dependence is the definitive clue.
• Cassette fan blades are held by a retaining clip on the motor shaft. Normal vibration from years of operation loosens the clip gradually, allowing the blade to wobble at specific RPMs where the displacement matches the gap to the housing.
• Running each fan speed while listening narrows the source faster than opening the unit. Ask your technician to do this speed-sweep test before diagnosing compressor issues — it takes under two minutes and can save thousands in unnecessary outdoor work.
• If the rattle has been present for weeks, check for scoring marks on the fan housing where the blade has been making contact. Prolonged contact wears the housing and can stress the motor bearing from the uneven load, turning a minor clip issue into a more expensive repair.