I believe the source of your problem is the cable going into the house.
Here is a simple way to isolate the source...Your TV is already plugged in so thats fine...disconnect the cable box from the TV and turn the TV on without any thing plugged into it...do you have the hum? if not then do the following...
Without using the ground loop isolator, plug one thing at a time into the TV....cable first and audio last...if the hum comes back then that component you plugged in is the source of your problem.
I would bet your cablebox is the source, either via the cable connection to your TV or the audio connection to your TV from the cable box if that is what you have connected.
If that is the case I would suspect your cable coming into the house is not ground referenced with your house ground. To fix this problem the cable needs to be grounded at the same place/point that your electrical breaker box is grounded.
The very fact that you have hum on the line means that the next lightning storm could potentially damage your TV or Cable Box because of an improper ground. In either case you have damaging voltages present that will degrade your TV over time.
I believe you would be much better off finding and fixing the source of the problem then get your money back for the ground loop isolator and put that into a cable surge protector and a AC surge protector for your TV...they will both last longer if you do.
Good luck
PS: If you also have your computer connected to the internet via that cable then that as well is in jepardy of getting damaged...one more thing you should think about.
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