Let's pin down the terms. The demarc is the place of separation between the cable company's wire and your house's wiring. It's a "grounding block," which is an F81 built so there's a place to insert a wire, up to 12 ga or so, that is run to an electrical ground. The standard setup has the cable company wire coming in on the left, the house wiring going out on the right, and the ground wire, a green wire, going to a ground, which ideally is at the power inlet to the house. It can also be a ground rod, sometimes even a water pipe. It should not be the ground of an outlet, but sometimes that's done and it can work.

When you write, presumably "demarc cable" means the cable from the cable company and "internal house demarc" is the cable going from the grounding block to all the house cable wiring. Right? That is, there's a ground block somewhere with the "demarc cable" coming from the cable company and the "internal house demarc" going to the house wiring, and the hum stops when undo the house wiring from that grounding block.

Why all this tedious statement of what we already know? Because I've never seen the term "demarc cable" and "internal house demarc," and while it should be clear just what that means... well, read my signature and think about times when perfectly obvious things have been misunderstood.

What are the details of how you tried grounding the grounding block? Connect to the chassis of the power boxes. If those are nowhere near the demarc, a water pipe might work. If not, the cable company might have to move the cable drop to where they can connect to a good ground. That might not be possible.

But if you temporarily add a ground by running a wire from the demarc to the power boxes and the hum goes away, you will prove to the cable guy that the problem is HIS GROUND and he needs to fix it, no matter what it takes.