Book case makes strange sound.

Just now. I had just sat down at the computer. The strangest whirring growl came out of the bookcase here in the library, just to the left of me. My eldest dog barked at it. It didn't last long, 15 or 20 seconds. But here is nothing on that side of the room or in that bookcase that could make such a sound. There is nothing on the other side of the bookcase. It backs up against an exterior wall that sits beside an exterior open porch. I said it was a whirring growl, but it didn't sound mechanical. Actually it is difficult to describe. My grey schnauzer was definitely perturbed by it. I sit here looking at the bookcase which goes from floor to ceiling (about 14 ft) and is completely filled with books, and I try to figure what could have made such a sound. This is the same bookcase that sneezed at some visitors we had, coughed at us, had a sound not unlike a tin can being bounced down the inside of the case. Yet there is nothing around this case but walls and books. I once thought the walls behind the bookcase must be hollow and I tried drilling through to see. I used 24 inch bits but I encountered wall all the way. If someone out there has an ingenius method of locating hidden passages, please let me know. The house was built in 1857 along the old National Road which was the only road west when the house was built. It was a rip roaring period of history and the old bricks could very well have passages and hidey holes. I did find a small room behind a fireplace several years ago and a tight passage up the left corner of the house from basement to attic. Only a very slim person could have gone up that passage. So I figure there is possibly other such hidden rooms or passages. But I'm not able to locate any. This is, you remember, the house that ate our cat. Did it find a secret passage? If it led outside you would think someone in town would have seen it wondering around. As it was never away from the house, surely it would have remained close; especially at dinner time. This place is a mystery and I'm having no luck at solving it.