Glug glug glug.
In my main bath on the 2nd floor of my house, the toilet and sink drain through a main stack that runs down to the basement and up through the roof for venting. The bathtub/shower on the other side of that bathroom drains down to the first floor, where the drain joins a drain for a washing machine in a 1/2 bath/laundry-room immediately below, and then down into the basement where the line finally connects to that same main stack. All PVC.
When the tub is filled and then drains (wife likes to take long baths), I hear a loud 'glug glug glug' from the drain for the 1st floor washing machine.
Is this a venting issue? It seems to me like the tub should have a vent line going up into the attic and through the roof (or connect over to the main stack and vent from up there) instead of going two floors down into the basement before venting.