I bought a used Sanyo DS27930 television for a 2nd tv and there are faint whitish lines running through the screen when I watch [digital] satellite through the coax, also a steady hum. It doesn't happen on component or composite. The picture on coax also isn't as good as the other two. I never noticed the problem on my old 20" TV.
I've had one person suggest it was a bad tuner, while another suggested it's a ground loop. I'm not sure how to fix a ground loop problem on a satellite coax but I suspect that would be the easiest to try first.
Rick