"...a condition that causes the sufferer to occasionally experience a tremendously loud noise as if from within his or her own head, usually described as an explosion, roar or a ringing noise... Although perceived as tremendously loud, the noise is usually not accompanied by pain... The cause of exploding head syndrome is not known, though some physicians have reported a correlation with stress or extreme fatigue..."
Further explanation at Wikipedia. Reddit commentors offer some personal experiences, and this trenchant comment: "It may be helpful to reassure the patient that this symptom is harmless: 'Don't worry it's nothing dangerous, you just have exploding head syndrome.'