There's a kind of hum all over the world. But only some people can hear it. It turns out not to be aliens – though I don't see why aliens wouldn't hum as they went about their world-conquering. Sometimes it's from local factories, electricity wires, fridges, or in my case a nearby airport that emits a thunderous roar when they're testing engines. All these things hum officially and it's not hard to find the source, not as hard as tracking down the humming aliens. But in two-thirds of cases, there's no obvious cause, and it has been decided that if you can still hear the hum you're suffering from vicious-cycle over-sensitive hearing. I think we may be on the verge of a new syndrome.