Cassette Rattling at Certain Speeds: Loose Fan Blade Clip
An office cassette rattles at some fan speeds and not others. That narrowness is the finding, and it points away from the compressor.
By Team Snowflake | Updated 24 Aug 2026
Teaching case based on recurring Snowflake service patterns; identifying details are combined.
How this presents
A rattle that came on a few weeks ago and comes and goes depending on the fan setting. Staff sitting underneath notice it most. Cooling is unaffected and the outdoor unit sounds normal from outside.
- Setup
- Mitsubishi Electric Cassette4-6 yearsOfficeMuseum
- Misread as
- Failing compressor bearings, which puts the whole outdoor unit up for replacement.
- Check first
- The rattle across low, medium and high, before anything is opened
What else produces this
Running the fan through its speeds costs two minutes and splits the problem in half. A sound belonging to the indoor fan changes with fan speed. A sound belonging to the outdoor side does not, because the compressor runs at its own pace regardless. The shape of that relationship then says which indoor part.
A rattle present at medium and absent at both low and high is not a wear pattern.
A compressor running smoothly at every indoor fan setting keeps the outdoor unit out of it.
A retaining clip shifted on the motor shaft lets the blade sit fractionally off-centre.
Clearance that closes at one speed and opens again at the next is geometry rather than damage, and geometry is adjustable.
Why it happens
A blade sitting slightly off-centre traces a wider circle than it should, and how wide depends on speed. Slow, and it never reaches the housing. Fast, and centrifugal force pulls it true and clear again. In between sits a band where the tip just catches on every turn. That band is the noise, and having silence on both sides of it is what rules out a worn bearing, which would worsen with speed and never improve.
What resolution involves
Re-secure the clip and set the blade to run true, then spin it by hand to confirm no play remains before closing up. The test is all three speeds rather than the one that was noisy, because a blade set true is quiet everywhere and a blade merely nudged is quiet somewhere. Nothing is replaced. Light marks where the tip grazed the housing are cosmetic.
How to tell it worked
Quiet at every speed, tested through the full range. Confirming the fix only at the setting that was noisy proves the least useful thing available, because a blade merely nudged is quiet at one speed too.
Before you approve a quote
Noise at one speed and silence either side is a clearance
- Run the fan through every speed and note where the noise starts and stops. It takes two minutes.
- Wear gets worse the harder a part works. A noise that disappears at high speed is not wear.
- A sound that changes with fan speed belongs to the indoor fan, not to 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.