Haunted bookshelves`

     Sometime ago I related an incident whereas the bookcase coughed several times, No explaination was evident then or now. Last night I was working with the computer in the library. It was rather late, a bit past midnight. My wife had retired for the night as she had to get up at 5 am and go to work. She has a very erratic schedule. The dog was sound asleep on the rug directly behind me. All of a sudden a noise, exactly like an empty tin can crashing down a brick road, jarred me suddenly from my work. The dog awakened  lay growling and looking directly at the fourth section of our built-in bookshelves. As I was sitting parellel with the bookcase and not more than six feet from it I was in an excellent position to hear the direction of the sound. The bookcase was built into the library when the house was built in 1857. It extends from ceiling to floor (about 12 feet). After the coughing episode, I had drilled several test holes into the back of the bookcase looking for an opening, like a passageway. I found none. The bookcase is on a wall that is away from the street, and is on a wing of the house. I immediately checked the rear of the house where the library juts out. There was no sign of anything that could have made such a loud tinny sound.