Coughing

Last night I went up up our attic because I stored various seed sprouting trays and lights up there out of the way. While I was up there I distinctly heard someone cough. The attic in this old Victorian brick is large. It has narrow tall windows which we keep closed in cold weather. Extremely loud mufflers and sirens can be heard if they pass in front of the house. Other street noises do not penetrate our attic. Birds getting into our eves can be heard. Both of my loud mouthed schnauzers going at full pitch on the ground floor cannot be heard unless the attic door is open and even then they are not loud; you would almost have to be listening for them to hear them. My wife and both dogs were on the first floor when I was in the attic. My wife was working in the kitchen and our dogs would have been confined to the kitchen, library, dining room area. So who coughed? Brings to mind the night we were entertaining a guest in the library and the southern wall bookcase coughed, quite distinctly and loud. It was snowing that night and my guest insisted we were being snooped by someone. I said you can't hear people from outside unless the windows are open (they were not) but he insisted so we went out into the snow around the library. There were no tracks but our own. Unless bats cough loudly, I have no idea what or who coughed in my attic while I retrieve gardening supplies.