You use your toilet several times a day and everything seems fine. Suddenly, something goes wrong even though you haven’t changed anything. From one day to the next things just break or wear down and the results of this wear and tear manifest themselves in inexplicable behaviour or noises. A loud clunk after the toilet flushes is a phenomenon called “water hammer.” It is a consequence of a sudden cut off in water, or may be the cut off mechanism itself.
There are several causes for a loud clunking noise caused by the toilet flush. Investigation can be confused by the ability of sound to travel along pipes. The location of the noise is the prime indication of its cause, but you have to be very precise and work out whether the noise originates from a location or whether the sound travelled there from somewhere else.
The easiest problem to solve is when the stopper inside the cistern is the cause of the clunk. When you press the level on the toilet to flush it, a plastic line connected to the internal part of the lever tightens up and pulls out a stopper at the other end. This stopper blocks the exit from the cistern to the pipe that leads down to the toilet bowl. When the stopper is closed, the cistern can fill up and retain sufficient water to flush the next time. When the stopper is open, water rushes out of the cistern, dragging down the ballcock, causing its lever to open up a valve letting water in. The stopper eventually floats back down to the closed position. If the stopper is quickly pulled by suction, or fits badly, it can make a clunk noise as it closes.
If the clunk seems to occur on a floor below the toilet or in the basement, the cause of the clunk is a sudden load on the drain pipe. A rush of water containing solids may jostle a badly secured pipe causing it to knock against a nearby joist, stud, nail or pipe. This will create a clunk sound. Fixing more brackets along the length of the drain pipe will hold it firmly in place and stop it knocking.
The third possible cause of a clunk noise after the flush is a valve on the incoming water pipe. That may be the valve inside the cistern --- the one controlled by the ballcock. If this is the case, replacing the ballcock assembly will solve the problem. If the valve is built into the pipework, you will need to call a plumber. The valve may not be directly related to the toilet, but may be somewhere else in the pipe network, such as on the water tank, or protecting a water heater or shower unit. Tracing the valve can be a difficult task, but it is probably worn and therefore over-reacting to a sudden change in water pressure.