Hear is an explanation from the link below

In the home water hammer often occurs when a dishwasher, washing machine, or toilet shuts off water flow, resulting in a loud bang or banging sound. A hydro pneumatic device similar in principle to a shock absorber called a ‘Water Hammer Arrestor’ can be installed between the water pipe and the machine which will absorb the shock and stop the banging.

When I flush my toilet, it makes a sound like the “end of the work day” whistle in the opening credits to the Flintstones. So it sounds like you and I may have the same problem. Unfortunately I have no idea what the cause is.