Both my toilets bubble when someone is taking a shower more than 5 minutes and if you flush while someone is in the shower the toilet will back up. Also once the bubbles start a few seconds later you will start noticing that the water in the shower starts to fill up and drain slowly. The toilets have been snaked and that helped for a couple of days and then it started up again. The vent on the roof has been snaked and we also put a screen so that no branches or leaves would go in (have a big oak tree in the yard). One time the toilet overflowed and we had 2 inches of water. The problem still occurred so we called in to have our septic tank emptied that seemed to help for a few weeks and no we are starting the problem all over again. What else can we do?