IF the heating system ductwork is not balanced, when the fan runs it can create a vacuum which can pull in sewer gases from leaks or possibly even from the roof vent. There could be a drain in the crawl space for the evaporator of the a/c unit and/or for the furnace if it is a high efficiency condensing one, and when it doesn't run, the trap could dry out (if it even has one), and allow sewer gasses to be drawn into the ductwork. Sealing all of the seams in the ductwork may help. Don't use duct tape - use the foil tape instead. It is sold with paper release one one side you pull off to expose the adhesive. The ducts must be clean before you install. My unprofessional opinion.