You'll have to troubleshoot where the noise comes from. Try to download a fanspeed control program, such as FanSpeed. And slow down some fans, till you know what fan makes all the noise. If this is the CPU or the GPU, (which probably have automatic fan-speed) you'll need to check temperatures first. If not, it's probably one of your casefans. You will need to let them runner slower, this can be done by setting speed manually in a fanspeed program (If the fan is connected to the motherboard). If not, you'll have to connect the fan to a lower voltage..
You can also buy a fanspeed reduction cable.. which is basicly an extra part attached to your cable, blocking some of the power.
Sorry, made a typ-o.. I meant "SpeedFan" as the program