We have a horrible odor in our unfinished portion of our basement, it is like a motor oil or 'shop' type smell. Our furnance / ac was looked at and we were told it wasn't that. It isn't our sump pump, washer or dryer. We had the floor drain snaked yesterday and the smell went away for about 18 hours. Now it is back and just as strong. Any ideas on what it could be and how I can get rid of it??