The Invisible Republic

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.

I never knew this had a name, or was so (relatively) common. It happens to me a couple of times a year.