i have a 440 CFM and a 170 CFM fan and they both sound like jet engines, the 440 being louder.
I don't know of any quiet fans (you may have better luck) but some things i've heard that may work to quiet them down (haven't tried it but they sound reasonable):
- use rubber grommets/washers to mount the fan
- use bungie cords to mount the fan
- use straight through duct work rather than the corrugated/collapsable kind
- limit the amount of duct work the fan has to push/pull air through