Why is my toilet gurgling?

What the noise actually is

A drain works because water going down is balanced by air moving through the vent. When something partly blocks the pipe, water cannot pass freely and air gets pushed or pulled through the nearest water seal instead. That is the bubbling you hear.

It is not the toilet being old, and it is not air in the water supply. It is a restriction somewhere in the drain.

Working out where the problem is

• Only the toilet gurgles, only when flushed: restriction in the branch behind the pan
• The toilet gurgles when the shower or bath drains: blockage in the line they share
• The shower gurgles when the toilet is flushed: same thing, other way round
• Several fixtures gurgle: the main sewer line, act on this one
• Bowl water sits lower than usual: often a venting problem rather than a blockage

Why it is worth acting on early

Gurgling is the stage before backing up. The restriction is partial, the pipe is still passing water, and clearing it now is a straightforward job.

Left alone, the same blockage keeps catching solids until it closes off entirely, and by then you have sewage looking for the lowest way out of the system.

What not to do

Do not reach for supermarket drain cleaner. On a partial blockage in a main line it rarely does anything useful, and it leaves caustic liquid sitting in the pipe.

Do not keep flushing to "clear it". If the line is restricted, all that does is push more into it.