Two options here that are simple/easy.
Run two fans in series. You'll end up with about 6V across each if you have 12V main rails.
Or stick a small resistor in series with one fan.
Keep an eye on the heat sink temps as you drop fan speeds to find the right balance.
Also clean your fan - dusty, caked fan blades are noisy.
Did you use a separate power supply for your fans or did you run them of your drivers and regulate them some how?