If you absolutely need the quietest fan that can move some good amounts of air, the only solution I have found is the Panasonic Whisperline. Not the most powerful, or best bang for the buck, but the quietest I've ever heard. About ~ 60 decibels

Panasonic FV-30NLF1 WhisperLine™ 340 CFM In-Line Fan (1.7 sones) [FV-30NLF1] - $160.00

^I use that one to suck from the scrubber, and then blow through the cooltube. The loudest part of it is the actual air moving. The fan is virtually silent in the room.

They make a less powerful, cheaper, quieter 4" version and then for a few bucks more theres an 8" 440cfm, which I plan on using in my next room