Even the latest, most energy-efficient and sleek toilet models suffer from the same problems as older toilets. Water can leak and spill from a number of different connections in a toilet, as well as bubble. A toilet that bubbles when flushed could be signaling a failing component or a problem with the sewer line.
How Toilets Work
Most toilets work simply through the force of gravity and water pressure. Water from your dwelling enters into the tank through the toilet fill valve. A float in the tank cuts off the fill valve at the appropriate level. The handle on the exterior of the tank is connected to a flapper in the bottom of the tank. By pressing the handle, it pulls up on a chain attached to the flapper. The water inside the tank rushes out of the tank bottom and into the toilet bowl. Outlets in the top of the toilet bowl give the toilet its characteristic swirl, which helps the water power waste inside the bowl into the sewer.
Toilet bubbling results when air is trapped in the water system of the toilet. Air can be introduced into a plumbing system in a number of ways. In a water system, air will naturally rise to the highest part of the water surface. In a toilet this could be the bowl or the inside of the tank, depending on where the air gets trapped.
Bad Fill Valve
A fill valve that is malfunctioning won't fill up the toilet tank properly and may make a bubbling sound when it tries to do so. The best way to check for this is by removing the toilet lid and watching the water around the fill valve as the tank refills. You should be able to hear or see the resulting bubbles. Fill valves are inexpensive to replace. Don't attempt to repair a fill valve by replacing parts inside, as some older models may not be compliant with the anti-siphon municipal codes for water fixtures in your area.
Sewer Line Blockage
If the bubbling in your toilet is coming from the bowl during a flush, this might be bad news, as the bubbling could be the result of a blockage in the sewer line. This blockage might, however, just be in the bowl itself. This you can test by running a closet auger through the bowl and into the flange beneath. If that stops the bubbling, then the block has been successfully flushed out. If the bubbling continues, the problem could be further up in the sewer main. These types of problems need professional inspection, as professionals have tools not available to the home owner, such as underground cameras.