Like many, my interest in this subject has roots in a personal experience, Here is mine:

When I was 12 years old, something invisible walked up our stairs one night. I was standing at the top of the stairs when it happened. The footsteps hit hard, like heavy boots. I could hear the hobnails in the soles of the boots (my grandfather was a shoe cobbler, I knew this sound) clink on the metal edges of each tread. I could hear each tread squeak slightly as the footfalls landed. I could hear the slight scrape of a heel with each one. Including the landing, there were 13 steps in the staircase (I knew this from playing with Slinkies on the steps). There were 13 corresponding footfalls, the last one booming right next to me where I stood. My point of view was looking directly down the stairs. I believe this sound came from something striking the stairs from the outside, it didn't come from underneath. Our house was over 80 years old, was not badly settled, we had no off-level surfaces or out-of-plumb walls. The foundation was slab. The void under the staircase was accessed through the utility room on the first floor. It was summer, no air conditioning or heating on. All others in the house were in bed asleep. I had not gone to bed yet nor was I sleepy; it was not a lucid dream. When the footfalls reached the landing, I experienced nothing more. I ran down the stairs and into my parents' room, adjacent to the staircase. My mother asked my what the matter was. I told here about the steps. She told me she heard them, too. I told her I didn't believe in ghosts. She just looked upset and wouldn't talk about it any more. I wrote it down that night so I wouldn't lose the details. Years later, they moved and the house was torn down. That was 33 years ago, and I still listen for sounds when I go to sleep at night.