Perhaps this is what the "looping of the earth wire" refers to. We call it an "earth loop" in English. I've experienced this issue when recording music.
http://ru.wikipedia.org/wiki/%D0%92%D1%8B%D1%80%D0%B0%D0%B2%D0%BD%D0%B8%D0%B2%D0%B0%D1%8E%D1%89%D0%B8%D0%B9_%D1%82%D0%BE%D0%BA