(Also, I once went to a doctor as a small child because I kept hearing this steady beat when trying to sleep. Well, when you hold a pillow to your ear, you can hear your own hearbeat...)
Sometimes I'm almost excited about growing older and losing that range of hearing.
Then I discovered it was from the electrical apparatus.
I actually have to get quite isolated to not be able to hear anything though. Not that I especially try to, although I do tend to disconnect my power extenders before going to sleep, just in case.