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?