Lexicon Rumming Loop

Electrical current in which an unwanted current (spurious current) flows whose frequency is related to that of the power system. A coupling mechanism (usually via a common impedance) thereby ensures that the interference signal couples into another circuit, which carries the audio wanted signal. Thus the interference is superimposed on the useful signal and becomes audible. Since the mains frequency in Europe is 50 Hz, the interference contains frequencies of 50 Hz and its multiples (harmonics). This manifests itself as humming or buzzing.

See also:

• hum in Wikipedia
• Knowledge thread in Hifi Forum
• Hifi Forum Questions about hum loops and hum interference come up very regularly, so a search of the forum can provide a great deal of useful information on how to fix them.