Cassette rattling at certain speeds: loose fan blade clip
The rattle showed up only at certain fan speeds, which made it sound like something deep in the system was failing. Worn compressor bearings were the obvious suspect. The team wanted to hear the pattern firsthand before naming a part.
By Team Snowflake | Updated 28 Jul 2026
Case summary
Mitsubishi Electric Cassette5 years oldOfficeMuseum, Singapore
- Concern
- Failing compressor bearings would put the whole outdoor unit up for replacement.
- Found
- Fan blade retaining clip had loosened, causing the blade to wobble and rattle against the housing at certain speeds
- Key check
- Ran the fan at each speed setting and isolated the rattle to blade movement, not compressor or bearing vibration
- Result
- The rattle was gone on the same visit. The outdoor unit stayed where it was, with one clip re-secured.
What we were told
The office aircon started rattling a few weeks ago. The noise comes and goes depending on the fan speed. Staff sitting under it notice it most. The office is worried the outdoor unit is on its way out.
What we checked
We ran the cassette at low, medium, and high speed and listened for when the rattle appeared and when it dropped away. This check takes under two minutes and separates fan noise from compressor noise. A sound that changes with fan speed belongs to the indoor fan; one that stays the same comes from the outdoor side. The rattle tracked the speed, so we opened the cassette panel and checked the fan blade on the motor shaft.
The rattle was there at medium speed but gone at low and high. That on-off pattern does not match compressor bearing noise.
The compressor ran smoothly at every indoor fan setting, with no roughness or grinding.
The fan blade retaining clip had shifted on the motor shaft, letting the blade wobble at the speed that triggered the rattle.
What we found
The retaining clip on the motor shaft had worked loose over five years of normal vibration, letting the fan blade sit slightly off-centre. At medium speed, the wobble was just wide enough for the blade tip to graze the plastic housing on each turn, which made the rattle. At low speed, the blade did not move far enough to touch the housing, so it stayed quiet. At high speed, the spin pulled the blade tight on the shaft and clear of the housing. A noise that comes and goes with speed points to a loose spinning part, not a worn one. Compressor bearing noise would stay the same at every fan speed.
What fixed it
We re-secured the retaining clip on the motor shaft and set the fan blade to sit true and centred. We spun the blade by hand to confirm there was no play left, then ran the unit through all three speeds to check the rattle was gone at every setting. No parts were replaced, no refrigerant work was done, and the compressor and outdoor unit were left untouched. The housing where the blade had grazed showed only light marks and did not need replacing. The fix took under thirty minutes on site.
Outcome
The rattle was gone on the same visit. The outdoor unit stayed where it was, with one clip re-secured.
What this case teaches us
A rattle that changes with fan speed points to the fan, not the compressor
- If the noise comes and goes as you change fan speed, the cause sits on the indoor fan. The compressor runs steadily, so its noise does not track the fan.
- A loose fan blade rattles in one speed band and stays quiet above and below it. That on-off pattern rules out worn bearings.
- Before accepting an outdoor unit replacement, ask for the speed test. Two minutes can show whether the fault is a clip, not the compressor.
Questions this case answers
Cassette rattling only at medium fan speed: what causes that?
Does a noise that changes with fan speed rule out the compressor?
Is a rattling cassette worth repairing, or is the outdoor unit finished?
How can one loose clip make that much noise?
Ready to get started?
Tell us what’s going on. Symptoms, setup, photos, anything we should know. We’ll assess and come back with the right next step.