Sewage smell that comes and goes

A few months ago we had a sewage smell in one of our basement rooms. Never could resolve it, but it eventually went away. Now it is back and we don’t know where it is coming from. We have checked the seals in the pipes under the floor and they seem ok. The smell is only in the basement room at the front of the house. Just don’t understand where it is coming from. Who should I call to investigate?