There is a SYS_Fan2 header that is PWM/Voltage Controlled, but it is often problematic and will only run half speed.

This issue is common on all GIGABYTE Boards that I have seen, not sure why they do that? BIOS f7d allows PWM Slope CPU/Case fan control (If you download from Stasio's link at TT), which should allow you to correct the SYS_Fan2 speeds, but that is only in that one BIOS