I have the same problem with my toilet. It happens at night there is such a hugh bang it is frightening. This house is over 20 years, in good condition, but has a flushometer and not a tank. Sometimes I have to hold the lever for a few seconds before it flushes.

In most cases, noises that occur when a faucet is turned on or a toilet is flushed are caused by air in the pipes. Often, when the air "bubble" slams through the pipe, any part of the pipe that is not secured in the wall (brackets/braces) will rattle against the internal spaces.

When does the banging noise occur, when you flush of when the toilet stops refilling?