There really should be a "homeowner's" forum for this site-that way you could participate only if you wanted.
This kind of question isn't really taking the food off anyone's table. If they're not finding anything with the unit, check the building. Is this a nat gas furnace or electric? If jel would have listed his region, even as vague as region of state, there could have been more info- fr'instance, is this in a crawlspace? If so, are the ducts sealed well, is there a negative pressure in the building envelope from improper venting(exhaust, hvac, appliances)? Is there any makeup air on this building? If there is a negative pressure condition, it will be made up with whatever holes it can find. If the unit is in the basement with a water heater and laundry dryer, it will equalize through the vents(water htr if nat gas, furnace also, in a/c season can draw down chimney)