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Ground Loops are a form of a Faraday Loop where any metal that's interconnected to any portion of the loop can act as a pickup for any stray EMF which will induce a voltage on that Loop and create hum. 99.9% of hum and whine caused by ground loops is on the signal side and not the power side since the signal side has such small voltage swings any induced current can create substantial noise. But that noise doesn't increase with volume since the induced current doesn't change only the original signal does which can often drown out the injected noise.

It's all about having two different points in an interconnected system at two different potentials (two different paths back to the main ground), the difference in potential causes a difference in voltage which creates the inductive pickup through a conductive loop, it's even worse in buildings where the entire buildings electrical wiring can become part of that pickup and induce noise from anywhere. Any rotating (alternator) or fluctuating (AC Current) magnetic field will then create current in the loop, if that loop is of low enough impedance (good grounds but just slightly different potential) then the alternating magnetic field created by the MOSFETs in the HU could theoretically induce those currents in a modulated way that would carry the original audio signal.
 
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