I built a house (4 bed 3 bath) less than a year ago in central FL on a septic system. There are no trees affecting the system. The bubbling toilet is located on an outside wall beside a shower tub in a Jack and Jill style bathroom. It occurred as the tub was drained on a normal plumbing usage day. I checked the vent stack on the roof today and there were no apparent blockages. The vent stack is above the sinks only (some 7-8 feet away from the tub and toilet). The affected toilet also happens to be the closest to the septic tank. My construction photographs confirm there is no vent through the roof for the tub and toilet in this bathroom.