We just a bought a house. I understand that it has been empty for over a year. We have living here about a month. I smell sewer gas odor in upstairs baths(2) and in laundry room in basement. Sink and tub drains slow, sometimes I hear knocking in wall in when I used sink. I have added water to drain in basement floor to make sure p-trap is not dry. Flushed sump pump. Replaced toilets, o-rings, cleaned drainage pipes that toilets sit on. We have new p-traps in both sinks. Smell is not continuous. I noticed that when tub drains I can smell the odor in laundry room. That is also where water shut of. Sump pump. Water heater, furnace live. There is another bath in basement. I have not replaced anything in that room except shut off valves for sink and toilet. Could this be a venting problem?