From The Guardian:
Whenever I wake up it is there and it is unbelievably loud. When nobody else can hear it you think you are going nuts, and it just wears you down,” says Simon Payne, 55, from Cambridgeshire. Payne is a hearer of the mysterious global phenomenon known as the Hum. “I have been desperate to get away from it, so I have stayed with friends – and even moved house.”
The Hum is experienced as a consistent, low-pitched noise, much like the sound of a large truck idling in a nearby parking lot. Hearers tend to report experiencing it in urban areas – leading some to conclude that it is, in fact, a form of noise pollution screened from most people by the general city soundscape. It is said to cause symptoms that range from insomnia to headaches to dizziness. But because its actual source is unknown, it is impossible to discern its effects accurately.