>typically devices that messed with magnetic fields at frequencies in the ultrasonic range. (Say, many switching power supplies.) Something a little bit loose w

>typically devices that messed with magnetic fields at frequencies in the ultrasonic range. (Say, many switching power supplies.) Something a little bit loose wiggles in the field and you get an ultrasonic whine. High-frequency sounds can end up driving nonlinear oscillators that emit lower-frequency sounds, and all magnetics wiggle - solid elasticity is enough looseness. If I had a whining switching power supply that would be my guess. Of course, some switching power supplies are simply designed to switch at 20khz...