In an electrical system, a ground loop occurs when two points of a circuit both intended to be at ground reference potential have a potential between them. They exist for no other reason than to make your life a living hell.
Let me show you two devices that will exorcise that nasty little gremlin so you can live that hum, buzz, and distortion-free lifestyle.
Devices that work:
USB Voltage Isolator
Pyle Mini Hum Eliminator