Strange sound

     This has been going on for a week. I hadn't mentioned it as I was sure I'd locate the source. For a couple of days my wife was insisting that when our furnace came on the water pump (we have hot water heat) was making a terrible sound like it was breaking down. I didn't hear it, and dreading going into our dank, dark, dungeonlike cellar, I put off going down there. Then, while using the computer one evening I heard what she was talking about. Figured I'd better get the deed over with because a furnace failure in these parts, especially on a cold night could be disasterious. You can only get a repairman out here between 8am and 5pm on a weekday; if you're lucky. So I went to the basement and awaited the furnace to kick on. It did, very smoothly and quietly. A few minutes later the hot water heater came on, also quite quietly. There is nothing else in the basement of a mechanical nature. I was relieved the appliances were working properly but it still didn't explain the noise. Then we discovered two things about the strange sound (1) It can only be heard in the Library (2) It appears to be timed. Last Tuesday we heard it at 8:04 pm,9:02pm,10:02pm and 11:02pm. Then on Wednesday we heard it at 6:02pm,7:02pm,9:02pm,10:02pm and 11:03pm. I went outside with my older dog and walked around the house to the library windows just before 10 and waited until 10:10 to go back inside. My wife heard and recorded the sound; I heard nothing like it outside. I tried again at 11pm with the same result. We may find a logical explaination yet; but this sound is frustrating. I think I'm going to set the recorder up in the basement under the library and see what, if anything is picked up.