Years ago when I replaced the ceiling fan in my smaller bathroom I did quite a bit of reading on the internet. I was interested in one in particular because it claimed it was almost silent. Nice. I don't remember why I didn't end up with that but the thing is, for that unit, the fan itself was actually mounted to the roof or wall, NOT in the ceiling, so the sound level was waaaayyyy lower.
I'm sure a Google will turn it up for you.