There is a fan button on the OC panel for changing the fan profiles between silent, normal, and turbo in normal mode. That said, I usually just set mine in BIOS for turbo and the last three on the list to Duty at 90% as its quite hot in my part of the world in the summer. Even with the fans in this config, the machine is pretty quiet. I am on the Maximus VI Extreme with 0804 BIOS.
Asus M6E | i7 4770K | 32GB Kingston HyperX RAM | 512GB Samsung 840 Pro SSD | GTX 780