Often the problem is not a lack of grounding, more that there is more than one grounding point (your cable TV entry point in this case). It is not uncommon for actual earth (the dirt) to have slight voltage differentials due to electrical leakage from other grounded equipment that has electrical leakage.

Yes, the cable TV coax should be grounded to something proper at the entry point. But that ground is not ‘canonical’, it is merely a conduit to discharge lightning and other possible dangerous potentials.

For audio equipment, the equipment grouping is effectively is own reference. All the equipment must agree upon and share the same ‘ground’. To do otherwise invites problems.

For electrical safety, the shared equipment ‘ground’ must also be connected to a ‘ground’ which is linked back to the electrical system ‘ground’. This is for electrical shock protection, mostly. Not for audio noise elimination.

You may see suggestions regarding separating the audio equipment’s ‘shared reference’ from the electrical system’s safety ground, sometimes called ‘lifting’ the ground (removing the third pin on a power bar cord, for example). While this may, in some cases, resolve the hum noise, there is risk of creating a safety hazard.

Regarding the coax isolator, I suggest installing it at the cable tv box, not the entry to the building.