I had a similar experience. I was pretty young, I'd say maybe 8 years old. I was reading a library book called The Lion, The Witch and the Wardrobe. At that time I'd check the max # of books out of the school library at any given time w/o parental knowledge. They couldn't keep track of the books anyway because I just inhaled them. Anyway...
My habit was to read in bed before falling asleep. My bedtime was early. I started to hear loud breathing that wasn't my own. It kept happening night after night. I got scared and mentioned it to my jw mother who said it was demons because the book was about a witch. I had to stop reading it right away.
I can't be sure what caused that noise, but I suspect it was my dad snoring on the floor below. There was a heat vent directly above where he was and another right by me, and I could clearly hear noises from that room.