[youtube http://www.youtube.com/watch?v=4YM1iwC6vhg?wmode=transparent&autohide=1&egm=0&hd=1&iv_load_policy=3&modestbranding=1&rel=0&showinfo=0&showsearch=0&w=500&h=374]
Even though this is demonstrated with a guitar amp, the principle still holds to other audio and electrical systems. Got a hum? Might be a ground loop.
(Source: http://www.youtube.com/)