Copied/pasted directly from some site that now i cant remember.
anyways, i thought this might help some of you that are having buzzing hissing or noise from power
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This is generally cause by a power connector that's plugged in backwards or a short somewhere on the motherboard. (with AT ps/mobo, check to see if those 2 power cords to the motherboard are in the right order. Another thing could be a problem with a short between the case and the motherboard. In this case, try using plastic standoffs or nylon washers between the screws holding it down to the case.
If the buzzing still persists, unplug the power connector going to the motherboard and try running the power supply by itself. If the fan spins on the power supply and no more buzzing can be heard, its most likely your motherboard. However, if the buzzing still does not stop, try replacing the power supply.