Get a cable isolator from Audio Advisor ($99) or make your own for about $4.
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This question has come up numerous times. Here is a copy of my solution which I have posted before:
I had this same problem. Here are the two parts you need, total cost $11:
Connect the cable coax wire to the transformer/splitter.
Connect the wires from the transformer/splitter to the 75-ohm coax combiner/splitter.
Connect the 75-ohm coax combiner/spltter to the input on your TV.
This was done for me by the cable TV guy. Worked like a charm. Why? I don't know. I guess it breaks the ground loop.
It is best to use the simple solution of isolation with two cheap 75 ohm to 300 ohm transformers of the capacitor -- not transformer -- type back to back. You can chech this with an ohm meter. There should be infinite ohms between the 300 ohm side and the coax side. If your house wiring is connected to the cable TV company's ground and not hardwired directly to your power service ground, you have set up alternate paths for lightning all through your house ( besides the hum problem ). Lightning will seek the path of least resistance. Then if the cable company's ground is better than your own service entrance ground, lightning will go all through your hose and out the cable to ground.