When I was little and would lie down in bed to take a nap, I often remember being able to hear the sound of my heartbeat in my ears. (thud, thud, thud) It's a sound I don't hear now. As a child I would close my eyes and imagine someone walking on an endless sidewalk in my head, usually a cartoon character from Sesame Street.
David walks on the treadmill in the morning. Our treadmill is in the garage, which shares a wall with our bedroom. While trying to get a few more minutes sleep in the morning I could hear the familiar (thud, thud, thud) from so long ago. Only this time it's not in my head - someone really is walking on a path that never ends.