I've always wondered that too. In very quiet rooms I hear quite intense ringing. I have distinct memories as a young child of wanting a fan turned on in my bedroom at night because of the sound of silence was ringing. It seems like what people describe as tinnitus, but... I don't seem to have any of the hearing loss that is always described as causing tinnitus. I can quite clearly high pitched noises such as CRT monitors and ultrasonic mosquito repellent. I don't have a history of working with loud equipment or going to concerts, and in any case that doesn't explain why I would hear it in childhood. It all doesn't quite correspond with with a lot of the causes and symptoms I hear described.