I recently installed a new bathroom fan in our master bath in hopes for a much quieter environment. I installed a NuTone Model # AERN110 (110 CFM, 1.0 sone).
I've never had an ultra quiet exhaust fan before so I wasn't sure what to expect but I wasn't impressed, it was still pretty loud. I just figured that was normal until I visited a friend and noticed how quiet his bathroom fan was. His fan was 6 years old and almost double the sone rating of mine.
I decided to reach out to NuTone technical support and after asking me a few questions decided to send out a replacement motor. I just replaced the motor and it sounds exactly the same. I'm assuming this means the problem in on my end but I don't know what to look for.
The fan is installed in the ceiling between the first and second floor (No attic access). It's connected using 4 inch flexible duct pipe directly to the fan (No reducers or elbows). I don't know the length of the duct but I do believe it vents to the outside but I'm not positive. One other detail that may or may not be important is when the fan is running I can hear it (very quietly) in the other bathroom located on the same floor.
Hopefully I've provided enough detail for someone to point me in the right direction but please feel free to ask questions and I'll answer to the best I can. Thanks for reading!