One of the two toilets in my rental house is bubbling when the shower or sink in that master bath area are on. In the past, about 6 months ago, I have had plumbers and rotoruter out and they could find no blockage in the line. Before I spend more money on an out of state house, could someone please tell me if they have suggestions?
It sounds like the toilet, shower and sink share a vent. This is pretty normal; no plumber in his right mind would run separate vented stacks for each drain in the house. The drains are instead tied into one vent stack, and then stacks are combined as they flow into the main sanitary drain. However, the shower or sink may be upstream of the toilet, and are pushing air in front of water which might be finding relief by bubbling up the toilet's drain. The plumbing can still pass code, but the intent of the applicable plumbing code is to prevent a drain being too far from its vent, which causes air to get trapped "downstream" of water in the line, resulting in problems like this (and slow drains).
The design of the toilet may have something to do with it. Toilets, like other drains, have U-bends; for a toilet this has the dual purpose of keeping water in the bowl, and also keeping sewer gases from pushing out into the room (similar to J-traps on sink'shower drains). However, "low-flow" toilets which use 1.6GPF or less are often designed with a shallower U-bend, so that it doesn't take as much water flow to induce the siphon that makes the contents of the bowl go away. Depending on other aspects of the design, like the relative order of the sink, toilet and shower in the drain line, water pushing past the tee to the toilet may be enough to force some air through the toilet's U-bend.
You may also have some issues with tee junctions in the plumbing. Specifically, I'm thinking of a tee joint being installed backwards. Drain tees are not true T-shapes; the perpendicular end instead curves into the straight section. The curve should direct water from the tee joint "downstream" towards the main stack, but if installed backwards it will force drain water (and air) towards "upstream" drains before gravity then pulls it back down the main line. This causes a backwash that slows drains, and yes it can force air in the drain lines past traps like the toilet U-bend. If the bathroom was ever renovated and the plumbing changed, and the work was not inspected (or the inspector missed the problem), this is plausible.
I sometimes have the same problem and the sewer backs up, but in my case the shower was placed where the bathroom sink was. The shower is after the toilet and closer to exterior wall. Whenever that happens I use the snake from outside cleanup and it take care of the problem. I am thinking to add a separate vent for the shower.