Bathroom fan too loud
Unfortunately the quietness is often determined by the air restriction. Yes a poorly made fan can rattle or the motor can hum or buzz but even the best fans with no hum no rattle no buzz can still make noise if the diameter of the outlet pipe is too small.

This is the problem with installers  

https://www.energyvanguard.com/blog/57709/How-to-Install-Flex-Duct-Properly

Flex duct for all its Glory is installed incorrectly 90% of the time if you're going to run a flex duct and leave it all floppy you better run a much bigger duct or just run rigid or install the