I dont have an antminer I have an Avalon 741 but I am guessing the sound is similar. When it gets warm it sounds like a vacuum cleaner however when it is very cool it is quiet. I moved mine into my shop and it was loud out there during the summer but now it is cold outside it is surprisingly quiet.
I dont have any ideas off the top of my head but if you could get it really cool it would cut down on the fan speed and noise dramatically.