 So the fact that you get this gurgle to me sounds like the venting is obstructed. That's where the air gets back into the system. And there's a vertical pipe that goes from the back of the plumbing connection up through the roof. And when it's starving for air, you get that type of a gulp that happens. So it could be that there's a partial obstruction somewhere in that drain line itself. Have you tried to clear that toilet to make sure that nothing was in there? Yes, my husband did take a plunger.