1. When you drain the water from the tub, that water travels into a very slightly sloped horizontal waste line and then travels through to the sewer or to the septic tank.
2. The waste line relies on a roof air vent, where air can exit as water approaches the air vent, and where the air can enter as the water passes by the same vent. So that pressure does not build up, and also, so that a vacuum is not created, this would slow the flow of the water down or stop it from flowing all together.
3. However; if there is a clog in the waste line, some distance from the toilet, where the water is either stopped or slowed to the point that the water begins to fill the waste line, and the roof vent servicing the toilet is also clogged; a build up of air pressure will occur which will eventually find the path of least resistence, and if that happens to be the toilet, then large air bubbles will start coming out of your toilet while making a glub, glub, glub noise.
4. Sometimes a bird, mouse, or a rat may have entered your roof vent and died. The creatures often times swell up and create a clog in either the vent, the drain, or both the vent and the drain.
(Another scenario, to consider, would be that your septic or sewer is full of water; backing up into your waste lines, thereby preventing waste water from flowing. This, if it is happening; happens generally while it is raining, requiring your either calling the sewer utility company, generally a free service , or calling a septic/plumber, one that services septic tanks, which is something that will, more than likely, cost you.)
5. Generally hiring a licensed plumber would be the best thing in the above case, lines 1 through 4 above.
6. However; if you are daring and would like to attempt fixing this problem yourself, then you will have to do the following.
7. 1st, you will want to go outside your home and open your clean-out for the sewer waste line. This clean-out is generally within 2 to 5 feet from your home.
8. 2nd, look inside the clean out to see if it is dry or if it is full of water. Sometimes it is hard to see, so you may need a stick to swish around while using a flash light. If the clean-out is full, then there is a clog between the clean-out and your septic tank/sewer, which will require snaking out the waste line between your clean-out and your septic tank/sewer. Also, because your waste line, in the house was full of water, there may be waste that built up and created a clog in the house water lines as well.
a. If when you look inside the clean-out, you see no water, or if the water is hardly flowing; skip 8. and move onto 9., below.
9. Next, turn on the tub, and then, while the tub is running, look inside the cleanout. (If following 8. above; hopefully, you will see the water steadily flowing; which means your next step will be to clean out the air vent servicing the toilet in question.) If the water is not flowing, have someone flush the toilet while you are looking into the clean-out, while the tub continues to run down the drain. If after 5 seconds or so the water is not flowing or it is moving extremely slow; your next step will be to turn off all the running water and remove the toilet in question. Then turn the tub water back on, on medium speed and snake the line from the toilet through to the clean-out.
10. While snaking the toilet, have someone look inside the clean-out to see when the water starts to flow. Once the water starts to flow, they should run inside and tell you that the water is starting to flow; so that you can run the snake back in forth very slowly in the area where the clog is, to break the clogged debris up thoroughly. Continue snaking until the snake can be seen at the clean-out. The water should be flowing pretty smoothly, even though the air vent is still clogged.
11. Next, purchase a 1" to 3" drain unclogger from your local supplier, one that can be hooked to your hose, or use a snake to snake the air vent out while running the tub water on medium speed. If you use the drain unclogger, screw it onto the hose and slide it inside the roof air vent servicing the toilet and tub in question. Slide the hose unclogger into the vent, far enough in that it does not touch the lead roof flange. Have someone turn the hose on while you hold the hose connected to the unclogger inside of the vent. The unclogger will swell up and completely seal the vent, and at the same time; it will fill the air vent with water and pressure, which should force the clog out and into the waste lines. Have someone look inside the clean-out while you are cleaning the roof air vent out. They should see chunks of something flow by when you have completely unclogged the vent.
12. Once the vent is unclogged, turn the tub on full speed and flush all the toilets in the house, while someone continues looking inside the clean-out. If the water moves rapidly, and after some time passes; does not stop or fill the clean-out up. Walla, you’re done.Because the above involves quite a lot of work and sweat as well, you should be really proud of yourself, and should take your right hand, reach it over your shoulder, and pat yourself on the back. You deserve it.