Strange sound repeats

     Last night the sound repeated at 9:59. I went to the basement five minutes to 11:00pm. There was no sound registered, I figured it didn't repeat. When I came upsdtairs, my wife said "Well, what is it?" I replied "You mean you heard it up here?" "yes", she said, "Didn't you?" No I hadn't and whats more the flour I scattered on the floor of the basement was totally undisturbed, not even mouse tracks. The sound is quite audible in the library but no where else inside or outside of the house. Yes, thats weird.