If you're talking about humming over the audio, it's a known issue with certain revisions. A lot of the time it's crappy cables, particularly if you're using RGB cables or newer generic AV cables. If you're using RGB you want to use the high quality extra grounded why-is-this-so-expensive cables, it doesn't totally eliminate it but it helps a great deal. If you have a Sega CD use the audio jacks on it instead.