http://www.coasttocoastam.com/show/2014/05/30
http://en.wikipedia.org/wiki/Electronic_voice_phenomenon,
http://paranormal.about.com/od/ghostaudiovideo/a/aa101507.htm
http://www.mindreader.com/html/opi/apparitions/index.htm
Sometimes its not just limited to being a case of ones imagination.
http://paranormal.about.com/od/ghostsandhauntings/a/Ghosts-What-Are-They.htm
http://curiosity.discovery.com/question/ghost-proof-existence-after-death
Former life forms that have become energy forms can be very tough to prove.
http://monkeywah.typepad.com/paranormalia/ghosts_and_apparitions
There are so many reports of haunting, but so many people are still unaware or just don't care.
http://theshadowlands.net/famous
http://paranormal.about.com/od/poltergeists/a/Poltergeists-Three-Famous-Cases.htm
http://www.real-british-ghosts.com/famous-hauntings.html
http://www.tlc.com/tv-shows/long-island-medium/games-and-more/10-famous-hauntings.htm
http://www.crystalinks.com/disincarnate.html
Some of these entities might never have been human to begin with.