Just put together a computer. When I turn on the power supply (rocker switch on the back) alone without ANY connectors connected, it doesn't make any noise. But when I connect the ATX power cable to the motherboard, and then turn the power supply, it starts making a very annoying hissing sound, and it's fan doesn't spin. The power supply is 400W, and the motherboard is Asus A7V8X-X. What can possibly be wrong?