This all occurred years ago when I was still in high school.
A little background: My grandparents live in an older neighborhood. Their house was built in the 60's or early 70's, I believe. This story takes place in their basement/garage and upstairs hallway.
Their basement has a guest bedroom with a bathroom that has a door leading to both the bedroom and the foyer with the door to the garage.
I had come to their house one day after school. It was within walking distance from my high school and I enjoyed the peace and quiet of their house. I was there alone and pretty hungry since it was after school. I walked down the stairs and into the garage to look through their freezer (located in the back of the garage). As I walked through the foyer with the garage door, I noticed in passing that the bathroom door was open. I walked into the garage paying it no mind (also leaving the garage door open) and made my way to the freezer.
As I was going through the contents of the freezer, I heard a "SLAM!" (it was extremely loud and made me actually jump) that startled me. I immediately put down what I was holding and went to check the foyer, which is where the noise originated from. The bathroom door was closed.
My initial thoughts were that someone may have broken in. I was home alone, so I ran as fast as I could up the stairs and into their office. I closed the door after me and started trying to calm myself. I had entered the locked house through the garage, and closed the giant garage door after me. I then assumed it may have been a draft, but the slam was so incredibly loud and earth-shattering that I knew it couldn't have been.
I remembered my sister (who lived there at the time) and grandparents would occasionally bring up the fact that weird things like this would happen sometimes because of a spirit that inhabited the home.
I was thinking and listening out for any other noises in the house, and started hearing loud footsteps outside of the office door. They have wood floors, and the footsteps were very heavy and exact. I gathered enough courage to take a peek out of the door and there was nothing there. I remember feeling a sense of terror and dread. I ran out of the house as fast as possible and waited outside until my grandmother came home.
I told her about it and later that night they all prayed over the house. I am a firm atheist, but I cannot explain what happened that day.