Unless you are using an indoor antenna, somewhere the cable from wherever is grounded. It is obviously not grounded at the same place as the electrical ground.

While newer electrical installs will have a ground rod stuck ten feet in the ground, I think in most areas cable and phone installers can still use a cold water pipe because those are more for lightning than ground faults in appliances, like if an internal hot wire gets shaved and touches the metal body.

The cable or antenna ground is so that if lightning strikes it does not arc across your house and kill you from ten feet away. It doesn't really protect the equipment much either.

There are ground gradients, I had a similar problem with an electician's house. He has a large house, so large that he decided to use two ground rids. We sold himm a video projectot which at the time was the only piece of his system that had a three prog grounded plug. He comes back and says that as soon as he plugs in the cable he gets a bar running up the screen. This is called a hum bar and had the exact same cause as your audio hum, but when the video gets the hum you get a bar. Our solution was to find an antenna isolator from an old hot chassis type TV and adapt it to be F to F rather than F to modified RCA. Finding such an isolator these days might not be so easy since TVs all when to SMPS type supplies and thus need no isolation.

You can actually build one, in case you would rather have that isolation transformer for other uses. If so, I'll look into the best/easiest way to do it, but if you are happy now and this is all academic you can just leave it as it is. However, your equipment is no longer grounded, which should not be a problem.