I am afraid that there are no magic cures here. In fact one of my reasons for sitting down to write this is that some people will land here first rather than on the website of a huckster aiming to steal money with quack devices.
If you live near the core of a major city, then you likely need no help with masking the Hum. The cacophony of urban life should generate more than enough ambient noise to mask the Hum. If you can hear the Hum through the din, then you are probably not hearing the Worldwide Hum that I am researching.
Most people find that a sufficient level of background noise suffices to block it. In my case, leaving the ensuite bathroom fan on is enough. For some folks, stronger white noise, or better yet, pink noise, works. There is a group of Hum Sufferers on one of the Yahoo Forums who report that a 207 Hz tone nicely masks it. The trick is to find the minimum level of ambient noise that just blocks the Hum.
The source of the Hum will be known before long, and for those who are suffering, I am sorry that I haven’t worked faster on this.