Fly-back transformers take an input voltage and drive them REALLY high. They are primarily used in CRT monitors and old style glass TV's. Oh and just a quick note Fly-back transformers are not normally in PC's and definitely not in ATX power supplies, at least none that I'm aware of. They can be charged up to certain level then discharged in certain ways to boost/chop voltage to a usable voltage/current. This is accomplished by using capacitor and Inductors. It will then switch and regulate the input voltage to give a steady 5v output. What does a switching regulator do? Well it allows a wide band of power, say 1v to 12v, to be put in to it. The reason for this is the regulator has to switch faster and faster to try and maintain a steady voltage at the output. As the voltage gets lower the noise will tend to get higher in frequency. The regulator continues to work even after power has been removed by sucking all of the juice from the surrounding capacitors. In almost all cases this is caused by a faulty voltage regulator that isn't shutting down as its supposed to. What is happening is one of the components is vibrating to the point of making sound, normally an inductor. The capacitors are powering it long enough to cause the noise. In fact there in almost everything electronic now. These are located throughout a computer in the PS, MB, HDD, CDROM, Graphics cards and more. What your hearing most likely is a faulty switching voltage regulator.
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