DUNBAR
Have that plumber perform a smoke test on that plumbing system. More than likely you have a leaking wax ring that has separated from the toilet or the closet flange that is causing this issue. This happens often. Aside from that, Have you made sure that you have ran water in all the drains that are not used on a regular basis, for example a floor drain in the utility room? When the traps on a drain dry up, sewer gas/foul air can pass into the structure. I would check these possibilities to make sure that isn’t what’s happening.
