This had happened to me back in the middle of the Summer, 2006 in my backyard. I had heard my dog barking in the middle of the afternoon and thought to myself, "What the hell could he be barking at?" It was weird and a bit alarming to me, seeing as my dog NEVER barked. So I had walked out on to my back deck and down the stairs to a patio we have and in the back corner of our backyard we have an old swing set.
I had peeked around the corner expecting to maybe see my dog barking at some stupid squirrel or maybe at another dog at the back fence. But what I saw made me start bawling. He was standing in front of this swing set while one single swing was swinging back and forth, back and forth as if someone were on it. Keep in mind it was the afternoon in the middle of summer with no wind. The swing showed no signs of slowing down at all. I had to run and get my mother and my little brother to make sure I wasn't going crazy. They both had witnessed the swing still not slowing down. I wish we could explain what really was going on. We had been having weird things happen since we moved into this house 14 years ago. Such as the lights flickering in our hallway, bumps in the night, things like that. But I have to say that seeing that swing freaks me out still to this day.