Cable cos. are notorious for bad grounding.
There should be a cable grounding block at the point the cable enters your house.
A ground wire should go from there to the ground system at the service entrance.
Those connections may just need to be cleaned up.
While the 'ground lift' devices do work, they aren't the safest way of ridding loops.
The main cable block has a new ground but the multi cable block after that does not so I put a ground out of a 220 line leftover from my oven hookup,from the multi block to the main block which then goes to the same ground as the service cable.However as luck would have it the comcast guy was across the street giving the neighbor a triple play package I told him about the hum and he walked over to look at the obvious,the same ground I just fixed,he said the ground was right and the piece I just put on is not needed but I'm leaving it.Well anyway he wouldnt come in the house to hear the hum since there is not a service call,
,but he gave me a brand new part he just put in his van ,he had 36 of these in an egg carton box and I got the first one,its a three inch cylinder,only writing on it is MDR made in china he said its for ground loop distortion I put on the cable at the wall then attached a 12 inch piece of cable with factory ends instead of my own hooked it to the back of the motorola digital box guess what the hum is gone...that simple I guess.