I used to get something similar when I was young, for years, and it was along teh lines of feeling like I could hear the television on when I was laying down - really it was the silence buzz, but I think our brains can trick us into hearing familar things when we are finally relaxing and laying down. If you only hear it as your trying to go to sleep or relaxing, I would say its just your brain tricking you - I would hear what sounded like a faint television on with people talking or radio or similar very familiar background noise, and sometimes as I was falling asleep you could make out words, like 'what', or just nonsense, that used to happen all the time, not any more as my life is so hectic now that when I finally hit the pillow I fall fast asleep. (everyone gets ringing in their ears, I would get it in one of the other once in awhile, lasts for a few minutes then fades)