Exploding Head Syndrome
"Exploding head syndrome is a condition that causes the sufferer to occasionally experience a tremendously loud noise as originating from within his or her own head, usually described as the sound of an explosion, roar, waves crashing against rocks, loud voices or screams or a ringing noise.
This noise usually occurs within an hour or two of falling asleep, but is not the result of a dream and can happen while awake as well. Perceived as extremely loud, the sound is usually not accompanied by pain. Attacks appear to change in number over time, with several attacks occurring in a space of days or weeks followed by months of remission. Sufferers often feel a sense of fear and anxiety after an attack, accompanied by elevated heart rate. Attacks are also often accompanied by perceived flashes of light (when perceived on their own, known as a "visual sleep start") or difficulty in breathing. The condition is also known as "auditory sleep starts." It is not thought to be dangerous, although it is sometimes distressing to experience."