I have a 10 year old house. The sinks in the master bath (top floor)have a strong sewage smell which rises from the drain when water is run. I installed drum traps but smell still rises. Two different plumbers have checked the vent stack for proper angle. No other fixtures in the house are affected (even on the same floor). It went from intermiitent smells in the past to constant now. My guess is that there is an obstruction somewhere in the vent (hornets nest?) Short of opening up walls and taking the stack apart, what can I do?