For "hum" read "resonate". Mechanical resonance means vibration but "in tune" with something.

So, an intermittent vibration of some sort could emanate from many sources and as I said much earlier in the thread, a process of elimination is needed.

If you've eliminated the sky box and are now looking at your central heating, you need someone else to switch the heating off and listen to the pump (which doesn't always stop running as soon as the heating goes off). There should be a switch to cut power to the boiler (and hence the pump) so use that and as soon as the pump stops running, get whoever switched it off for you to shout out while you listen in the room where you hear the "hum".

It's possible that sludge may have accumulated in one spot and, daft as it sounds a loose spot of solder could "vibrate"/resonate and make a noise. A flush of the system might cure it.