A ground loop is an electrical problem that can present as a noisy hum. Some musicians like to use a three-to-two pin converter plug as a quick fix for the problem, but that can remove the important safety features of using a grounded outlet. Start by plugging all of the equipment that will be handling your audio signal into a single power strip (be careful not to overload it!). This works because all of the ground lines in your gear will eventually lead to a single ground point, the wall outlet, eliminating any chance of a difference in electrical potential, which is the root cause of a ground loop.
You can read about other ways to avoid or fix a ground loop here: http://en.audiofanzine.com/recording-mixing/editorial/articles/banish-ground-loop-hum.html