You can adjust the fan speeds by editing the file /config/bmminer.conf and adding these lines:

"bitmain-fan-ctrl" : true,
"bitmain-fan-pwm" : "50"


That adjusts it to 50 percent the default speed

Here is the whole bracket in my current config:

}
]
,
"api-listen" : true,
"api-network" : true,
"api-groups" : "A:stats:pools:devs:summary:version",
"api-allow" : "A:0/0,W:*",
"bitmain-use-vil" : true,
"bitmain-freq" : "_ant_voltage=0706",
"bitmain-voltage" : "0706",
"multi-version" : "1",
"bitmain-fan-ctrl" : true,
"bitmain-fan-pwm" : "50"
}