Lower the room temp the fans wont run as fast there by will run quieter.
Make sure the miners are clean

4mos of running and i had to blow out the dust bunnies they were driving temps up.
I drool over this idea
http://www.grcooling.com/ they would be silent
I wonder if anyone has tried to submerse a antminer yet

The C1 is liquid cooled its a bit quieter. But the above link man that would be sooo sweet
.
my miner is new so there is probably no dust, and room temp is 18 celcius.