We have a similar problem. The smell seems like sewer gas but it is much worse when we run the heater. It seems to be coming from the vents. Can a plumbing problem seem to be coming from a air duct? Or possibly the humidifier? If you have a cracked/blocked vent stack, could you smell it in the attic? We can't, only in the bath and ajoining closet and bedroom.
Thanks for any help