Several years ago, the old metal drum trap was replaced under our original bathtub. The old trap and pipe from it to the plumbing tree was clogged from 50 years of use. Both the pipe and the new P-trap are PVC. Ever since, when the dishwasher or the washer drain, wwe can hear gurgling from the old bath tub, but not from the new one or any other sinks. Can you tell me why this would happen? Can/should the old metal drum have been replaced with one out of PVC or do they even make that? Could the P-trap not be sufficiently deep enough on the exit section to so it holds enough water to keep air from being pushed back? We do not get sewage smells from the bath tub, so I don't think the trap is dry. We're at a loss to understand this. Any thoughts?