This happened to me when I was about 15. Our house is set up kind of like a "T". We have the living room and kitchen on two sides and then the bedrooms down the middle. There is a door that opens from the kitchen into the driveway.
It was a summer day and I was home alone; I forgot where the rest of my family went. I just finished washing the dishes and I took out the trash. We put our trash bins on the grass next to the drive way. After I threw the trash away and made my way back to the house I opened the door and I took one step inside the house, still had my hand on the door knob, and I heard loud slow breathing; like someone taking in a deep breath then exhaling it slowly. It was coming from the living room. It was different from a human's breathing because it was way too loud.
I slowly closed the door and just went and sat outside on the drive way. I remember thinking to myself, just go in there but then again, what if I walk into the living room and there is like this THING sitting there, breathing... Turning its head to look at me! So, I just sat there for a while. I don't know why, but I remember checking, like 3 times to see of the breathing had stopped and it didn't.
For some reason I was still clam, although scared enough to not enter the house, I just went back and sat down and waited for my parents to come home. I did not tell anybody about it until we moved out of that house. I don't feel that it was demonic or anything, like I said I was very calm and wasn't horrified. It was a weird experience and I just thought maybe it might be a nice story to read, although it may be boring compared to the other ones on this site.