This problem is somewhat intermittent, happening most of the time but not always. I'll take a nice soak then drain the bath water, only to hear it gurgling up through a basement toilet and onto the floor. My house has one story and a full basement. The tub drains straight into the main drain line. The toilet is right below the tub and connects somewhere downstream of that. I've already had the drain line rootered from the cleanout to the alley, about 75'. There weren't any major blockages, just some small tree roots. I had the drain line inspected a few years ago and there weren't any issues with dislodged sections, etc. What next? Could this be due to a clogged vent pipe?