My guess is that it's coming from the tuner in the cable box. It could be similar to the loud buzz some systems make when there is a text overlay on the screen, especially white, or a large white portion of the video itself.
The fact that reducing the cable volume mad a difference leans me in that direction. I suggest that you try swapping cable boxes, and see if you can duplicate the problem. Maybe the cable engineers can help.
Larry
www.fineelectricco.com