[0] Acoustic noise [Switching power supply] Usually inaudible to most humans, unless they have a fan or are unloaded/malfunctioning, or use a switching frequency within the audio range, or the laminations of the coil vibrate at a subharmonic of the operating frequency. [Notes] The operating frequency of an unloaded SMPS is sometimes in the audible human range, and may sound subjectively quite loud for people whose hearing is very sensitive to the relevant frequency range. [Linear power supply] 
Faint, usually inaudible mains hum, usually due to vibration of windings in the transformer or magnetostriction.