What are the very lowest settings? If I push the boards down to 8V & 300MHz, the fans are at 1% (set min to 75C). This does about 6Th/s.

The noisiest part is the power supply fan. Easy fix is to replace for something slower or reduce the voltage, powering the fan - !!!BEFORE YOU DO THAT, MAKE SURE THE SUPPLY DOESN'T NEED THAT MUCH OF COOLING!!!