I had a ground loop that got introduced into my system when my cable was installed too. Comcast gave me a PVR and immediately thereafter, I got a hummmmm....
I fixed it with two items from Radio Shack, for less than $10. Someone else can maybe jump in here and help me identify what I bought, but essentially, the cable now goes from the wall, to an adaptor that you would use to hook up a cable line to an older, non cable ready tv (you know, with the little wires terminating in small spade plugs). This then gets hooked up to another adaptor that turns that signal back into a regular cable signal.
My hum is gone, but the reception is pretty poor on some channels, and I have a ghosting issue on the big broadcasts that can be pretty annoying - but it is less annoying than hum.
edit: I just found
this thread from the past that might provide some answers.
