We live in an old farm house with one vent pipe and the toilet bubbles when using the shower or bath. The septic tank was just drained a couple of years ago so I don't think that's it. I think it might be the vent pipe because the toilet wants to overflow if the tub is draining but otherwise it flushes fine. Could it be a blockage in the vent pipe? If so what is the best way to clean it?

The tub drain drains into the same pipe as the commode does, and as long as the water in the tub is not higher than the water in the commode it will not overflow unless you flush the commode while the water is that high. Using the lavatory is another thing, I have an idea that the lavatory drains into the vent for the commode and since the lavatory is higher than the commode it is possible to overflow the commode by running water into the lavatory.

I think the problem is a stopped up in the line from the commode to the septic tank. Or even though you had the tank drained two years ago it can still be a problem with the septic system. The tank could be full and if it is it will keep the field tiles draining all of the time and this can saturate the field lines to the point that it will not evaporate as fast as you are putting water into the tank.

Letmetellu gave you a excellent discription of your problem. Thje only problem with his answer is that he failed to provide a solution.

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the toilet bubbles when using the shower or bath. The septic tank was just drained a couple of years ago so I don't think that's it. . Could it be a blockage in the vent pipe? If so what is the best way to clean it?

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If you had a clog in the vent, (you do have a roof vent pipe above the bathroom don't you? And what vents the kitchen sink?) it would produce a gurgle. Bubbles indicate a clog in the drain line.
Check in your attic to be sure the kitchen doesn't revent in to the bathroom roof vent and snake from the bathroom roof vent. Put out enough cable to reach the base and 20 feet more. Good luck and thank you for rating my reply. M Tom