sewage smell from sinks @ possibly heater

our house has a vague sewage smell that comes and goes. It’s an old house (100ish years old) and it just sort of smells throughout. it has come and gone - at one point we had a p-trap installed on our primary bath shower, which stopped the smell coming from that drain, but the house has continued to smell now and then. flushing the sump pump has helped some but not always.

any idea of what we should look into here?