Marty, Have you got a dog,? if not, then get one, get one from a friend Im thinking about a terrer dog type... Go down to the basement or to the area where you think the smell is comming from and let the dog run around. What are the walls made of, brick,drywall,lath and plaster. find out how thick the walls are, ( to see if you have space behind the wall(s)) I still think you may have an animal dead in your house, even old plaster or rot of some kind. Come back to use on this please. John 2000.