By now, in most places around the world there are laws in place to mandate a minimum efficiency for power supplies. To achieve the mandated efficiency, manufacturers use so-called switching power supplies, which are based on quickly switching electronic switches on and off and using the energy "storage" characteristics of coils (current) and capacitors (voltage). Under certain circumstances, those switching frequencies or harmonics thereof (harmonics=base frequency multiplied by a whole number, but sometimes also divided) fall into the audible range. The magnetic fields changing over time cause forces changing over time on the metal loops of the coil, making them want to move in sync. If nothing prevents them from moving and if, by moving, they somehow hit each other or are massive enough to move enough air to be heard, you get the whine. Sometimes capacitors can also whine because of something called piezoelectric effect.

It can be alleviated by careful choice of switching frequency and coils and capacitors, although perfect silence in all conditions often isn't a top priority in design. Coating the coil can also help, but the used material needs to be as transparent as possible to the magnetic field and could inversely affect thermal dissipation.