Somewhere about 20 years ago, I was a newly married guy renting a very old house. It was supposedly built in 1906, though I can't verify it. I DO know it had a single heating vent in the center of the living room floor. It was probably about three feet square, as I recall, and cast iron. Directly below the vent was the oil heater located in the unfinished basement.
About once a week, my new bride and I would hear a CLANG from the heating vent. I originally wrote it off as regular expansion and contraction of the metal but then we noticed that when it would happen it was always at the same time: 1 AM.
I could replicate the sound by standing on it and shifting my weight, making the metal vent rock in place a tiny bit. So, the 'how' of it was answered, but that opened up the 'who' question. Who was walking through our living room at one in the morning?
One night I happened to be awake and watching TV in the living room when the vent rocked and make that sound. The oft mentioned cold zone moved past me as well, so I knew it was a ghost. I never did see anything.
A few months later, a friend of ours moved into the finished attic. After about a week, he came screaming down the stairs saying that someone was up there. He described a man who was 50-ish, balding and attired in average everyday clothes (nothing unique about him, really) who was floating over his bed, staring down at him with a look of curiosity.
I made the assumption that he just wanted to know who this new person living in the house was. Nothing bad had ever happened while we lived there, so I wasn't very concerned about it. Our friend moved out shortly after that, though.
This was a long time before the Internet and I really didn't want to go through piles of papers at the library trying to find out who this man was, so I just forgot about it. I'd search for it now, but I don't even remember the address.
Just thought I'd share the story, and let you know not all spirits are mad or vengeful. Some are just curious.