A ground loop.
Hold on, it's a bit complicated to understand, at first.
I think you have a ground loop
If you notice, your computer power wall plug has three prongs on it, two flat blades, and a round middle prong.
The middle (round) prong is the ground.
And your new TV has a three prong power plug too.
The computer is grounded to the power wall plug, but it is also grounded to the internet cable or coax, through the Ethernet cable.
The TV is grounded to the wall plug and to the cable TV.
Multiple grounds (instead of just ONE ground) cause the loop, and the loud hum or buzz. You may also notice a faint horizontal line (ghost image) in your TV picture, this is also caused by a ground loop.
At the hardware store you can buy a plug adapter, that changes a three prong power plug into a two prong power plug. This adapter "skips" the third prong (ground). It costs about $1.
This adapter was used to plug a 3 prong power plug into an old style 2 prong electrical outlet. But you are going to use it to disconnect the power ground from your television. In other words, using this adapter will disconnect the 3rd prong ground from the electric power plug on the TV.
Please use the adapter, and do not CUT the third prong off of the TV power plug.
Now that you have installed the power plug adapter, between the power outlet and the TV power cord, disconnecting the power ground (third pin) from the TV, the buzz should be gone.
Please note that these adapters may have a ground wire, or a ground screw terminal, which should NOT be connected to ground.
However, ground loops can be more complicated, and you may need to install more plug adapters, until just ONE of the equipment power plugs in your entire system is grounded, and the rest are all lifted off from ground. (TV, Amplifier, cable box, sub woofer, any equipment that has a three prong power plug, and is connected in your system, may need to be lifted with an adapter)
OR: Use a cable TV isolation transformer, this will prevent the cable ground from connecting to your theater system or audio system ground. The TV will still work, but the hum or buzz will be gone. This isolation transformer needs to be installed where the cable comes into your house, BEFORE the splitter, and the isolation transformer or the splitter should NOT be grounded with additional ground wires.
Now some people try to get the cable company to fix this problem (forget that), you are better off getting a CATV isolation transformer or lifting the power grounds (start with the TV and sub-woofer ground). No, grounding the cable TV where it comes into your house DOES NOT solve a ground loop in an audio system.