I have this. About once a month, in the early morning hours, I hear a hard to describe, "internal" sound, as if a hard object hit my cranium. Extremely loud, bu

I have this. About once a month, in the early morning hours, I hear a hard to describe, "internal" sound, as if a hard object hit my cranium. Extremely loud, but painless. 20 years ago it was distressing but now it barely wakes me up. Stress does seem to increase occurrences for me.