Can you notice that the smell is stronger in one room or location than another? This might help locate it. Do you have central air conditioning? During the winter, that generally doesn't run. If the condensate line has a trap, (it should) it can dry out in the winter, and that can leave a direct path to the sewer to allow gases to be sucked into the air ducts. If this is the closest room to the furnace, it could be from that. If the laundry drain line does not have a proper vent to it, the high velocity of the pumped water can suck a drain trap dry. Once a trap is sucked dry, there's nothing to stop gasses from coming through. A cold air return could suck that into the air ducts and distribute it around the house.