Are you fans connected to the motherboard, or are they connected straight to the PSU?
If they're connected to the MOBO, you can change the speed of them in the BIOS, or using something like Speedfan. You could also buy a fan controller, which you'd plug them into. That would allow you to control the speed of them with a dial, which would be a lot easier.
If you plug it into your mobo, doesnt the fan speed up when necessary?
or does it still run at full speed?