Every now and again, while wandering around on the internet, you come across the answer to a question you hadn't thought to ask. This happened to me the other day, when I stumbled on this passage:
As strange as the name sounds, exploding head syndrome is actually a rare and relatively undocumented sleep phenomenon. While sleeping or dozing, a person with the condition hears a terrifically loud sound in their head, such as a bomb exploding, a clash of cymbals or a gun going off.
My first thought was, "Oh, so that's a real thing, not just some weird and random peculiarity of my own." My second was, "And it has a really cool name: Exploding Head Syndrome. I have Exploding Head Syndrome."
The article explains the what, but not the why, probably because no-one knows what causes it or what the mechanisms involved are. There are many comments by readers on that page, most saying that they have experienced it, but never knew what it was, just like me. However, many were also frightened by it and thought it might be some dire harbinger of serious illness. For some reason (and for the life of me, I can't think why) it never crossed my mind that it might be a symptom of something serious. It didn't hurt, I didn't have any after-effects from it, and otherwise I felt fine: it was just my head going bang for some mysterious reason of its own.
In fact, it never really frightened me that much, though of course it does jolt you awake suddenly when it happens. Mine seem to happen when I'm just on the point of going to sleep, so perhaps it's clearer to me that it's not a real sound, but something inside my head. My sounds also vary a fair bit and are rather inventive, so it often makes me smile. For example, I get the standard bangs and crashes, but also a sound like a grand piano falling on the floor from a great height, and another that sounds very like the Mac alert sound 'Sosumi', but at a deeper pitch. I quite like the fact that I have my own alert sounds, even if they do wake me up.