A childhood belief about sleeping

Sometimes, as I lay in bed trying to fall asleep, a small scratching sound would keep me awake. The strange part about it was that it seemed to come from the inside of the pillow.
My first thought was that it was a small spider trapped inside the pillow and was scratching with one of its little spider legs. I'd punch the pillow, trying to move it way from my ear area. This sometimes worked and I'd go to sleep.
I was slightly nervous with having a scratching spider inside my pillow, but then realized that it didn't make sense. A bug wouldn't survive the occasional washing or trip through the dryer. I also noticed the scratching occurred on other pillows. For a time, I gave up trying to figure it out and was just content that punching the pillow enough times would quiet the scratching.
Later I realized I was hearing my own heartbeat. The shiny fabric of some pillow covers was reflecting the sound.