Noise in switching regulator
Under light load, certain regulators can enter PFM mode which consists of varying the frequency of a pulse. If the load is light enough, the regulator will only need to supply a few (comparatively) pulses per second. This can induce low frequency noise though I'd imagine the frequency is load dependent. 

The Microchip TC105 for example is a buck converter which switches between PWM and PFM mode for light load conditions - defined as duty cycles less than 10%.