Help with gurggling noise

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?

My guess is that the old drum trap was not vented and thus your new trap is not vented or the vent is clogged. Homes of that age, at least in my area, are generally plumbed with galvanized pipe. The insides rust and flake, the flaking can build up over the years to the point that it clogges the vents particularly on smaller pipe I.D. like 1.5".

When you run the water in the other fixtures the DWV system is trying to draw air but can't so it creates a vacuum and sucks the water out of the trap until it gets some air through it and you hear the gurgling.

drain

If there is no water in the trap, you will hear the sound of the washer discharge draining. If there is no vent, and you also have other problems to go with it, then the discharge could be sucking the trap dry causing the gurgling sound. We would have to be there and hear what kind of sound you are hearing.

Gurgling

Do you know if drum traps are available in PVC?