usually have a transformer on the input. They can start to sing after a while. Many of the capacitors inside the supply are the same as the mobo leakers. If some of them are bad, the power supply voltages could lose some regulating capability and let some of the load noise through. Kinda WAG.
Have even heard video card singing. There is no dangerous voltages in an ATX case, so [ground yourself first] and touch any touchable things to see if the ringing stops.
Getting a very irritating fairly low pitched hum from the computer. I temporarily stopped the Power supply fan, then the cpu fan, and made no difference. Reasonably sure it is not the HDD.
Actually the noise comes and goes a bit, and changes pitch slightly sometimes.
So question is, can a supply cause this kind of noise, even if the fan is not the problem?
Running XP with a 2 Gig P4 , 512M ram, 100Gig HDD, floppy, DVD drive, and CDRW drive. Power supply is only 300 watts, but have had no problems for 2 years, except this recent annoying noise. Motherboard caps all "appear" ok.