Ouch. Although when I first read the title I was expecting something a bit different.
Exploding head syndrome is a rare condition first reported by a British physician in 1988 (PMID 2899248) that causes the sufferer to occasionally experience a tremendously loud noise as if from within their own head, usually described as an explosion or a roar. This usually occurs within an hour or two of falling asleep, but is not the result of a dream. Although perceived as tremendously loud, the noise is usually not accompanied by pain.