Why is my bath exhaust fan noisy only when installed in the ceiling box?

Question

The house and fan are 45 years old. The fan in the guest bathroom sounded like a jet plane since we bought the house, but we finally decided enough was enough after some house guests complained. (Note: We really like these people. If you get unwelcome guests, do NOT fix your noisy fan!)

Took the fan out, plugged it into an outlet in the shop. Quiet, and I couldn't imagine a new one being quieter. Oiled the shaft anyway. Put it back. Jet plane again.

Why would the fan be so loud, but only when installed?

Answer 1

To me it sounds like you removed the back draft damper that may have gotten stuck.

Answer 2

Ever wonder how an acoustic guitar can be so loud even though it has only thin vibrating strings? It's the box.

Similarly, the housing and exit duct of a fan are the culprits with a fan. The vibrations are in effect amplified by this apparatus.

Even if one installs material to dampen the transfer of vibrations to it, they still get transferred quite efficiently through the steel screws by which the fan is installed. I don't know exactly what to do about this.

Answer 3

Obviously this fix won't apply in all cases, but it's something to check for.

Searched around for some answers, and noticed some advice about the exhaust duct being too small. With the fan out, I felt for the duct to see how big it was, and a piece of rubber came out, around the same size as the duct. Possibly something cut out for the duct 45 years ago that was inside the whole time. Or, maybe it came off of some sort of fitting.

Anyway, with the rubber piece in the trashcan and the fan back, it is QUIET!