All guesses here, of course...
Are you sure it's the plumbing?
Check that all drains have water in their traps -- including floor drains.
Check the air conditioner and/or furnace condensate drains. If they are connected to a drain, it must be indirectly (air gap or break) and then through a trap which is not allowed to dry out (not directly into a vent in the attic). If not, sewer gas can be drawn into the heating system. Probably not your problem since the smell is localized.