All you have to do to quiet a fan is two things.
1. A speed controller(10-30 bucks)
2. Ducting and/or filter. You can take any fan and turn it on full blast, add ducting to one side it quiets about 25%, and ducting to the other end to it quiets it 50%, a filter on one end and a length of ducting on the other quiets it about 50-60%. Most the noise comes from either the air going into the fan or the air going out, not the fan itself. So if you cure the in/out noise you can basically quiet a fan 50-75%. If you need ultra quiet you can build a box, enclose the fan in that box, seal box and then with ducting on both ends it's almost silent.
I have a total of almost 10 6" and 4" fans in two rooms in a basement and I can't even hear them until I'm all the way down the stairs in the basement.