Just came up with something while looking this up on Google:
•Passive PFC inductors in some power supplies. This buzzing sound is generated by low frequency viberation of magnetized cores of the Passive PFC inductor and has a low frequency.
•The switching stage circuit of the PSU. Because the switching frequency is usually above 50KHz in design, this noise is usually beyond hearing limits of human kind (20Hz~20kHz). However if a lower frequency viberation is generated in the feedback loop, you may hear it from the viberation of coils, tenuous but sharp sound.
•DC-DC VRMs are used on both Mobos and Video cards. Usually the viberation of inductors (or capacitors) can't be heard because of the ultra-high frequency(>100kHz I think), but if there's lower frequency resonance (<20kHz) for some reason, the lower frequency components can be heard.
•If there's no load on one of the PSU's output (12V, 5V, 3.3V, 5Vsb), the PSU is working abnormally and can emmit buzzing sound. Enhance uses a 20Ohm resistor as the load on 5Vsb output and calls it "standby noise filter". Clever design.
Not that I understand too much of it.
