Off and on I was having a sewage smell in the basement. Last time it happened I tracked the smell to a open pvc pipe sticking down from the ceiling. Not home currently but I think it was like 1 1/2 - 2" in diameter. I cannot easily trace it but I am assuming it is connected to the plumbing ventilation. Is this normal? I do have an older gas furnace I thought maybe it was to pull air in but it is very far from the furnace. I put some duct tape over it and the smell stopped. Can I permanently cap it off?