I had this same problem. I traced it to a ground loop hum coming from my cable line. I eliminated it by using a MIT ISO-Linq ground isolation filter. Mondial's Magic Box would probably have worked just as well. I've heard that Radio Shack sells such a device, but the 2 stores I went to didn't have one. Maybe you'd have to mail order it. One way to check is to disconnect your incoming cable or sattelite feed, and see if the hum goes away. If it does, then you'll need a ground loop isolation filter.
Good luck,
John
Good luck,
John