Why does my toilet sound like the QE2 when I flush it ?

Most likely cause

Water hammer. While many people incorrectly use the term water hammer to mean any loud banging in your pipes, it’s actually a specific phenomenon.

A water hammer occurs when a water valve is suddenly shut off. All the water that was running then crashes into the valve, shaking your pipes, creating the knocking noise you hear.

In some cases, water hammer can be violent enough to shake the pipes loose of their joints and cause leaks.

How to fix water hammer

The solution to water hammer usually depends on the age of your home.

Homes built before the 1960s usually have air chambers. Air chambers are basically T-sections of pipe that contain air and act as shock absorbers. However, over time the air in the chamber can become displaced by water.

If you have air chambers, you can reduce water hammer by draining all the water out of your home’s plumbing and then refilling it. Here’s how:

Shut off the water to your home at the main

Open the highest faucet in your home

Open the lowest faucet (it’s usually outside or in the basement) and let all the water drain out. At this point the air will be “refilled” in the air chambers

Turn the lowest faucet off (the one you opened in step #3) and turn the water main back on

Let the top faucet run until it stops sputtering, then turn it off

If this does work or if your house i after 1960s ask again and we'll try something else.

https://www.youtube.com/watch?v=e3_vK2NniYw

https://www.youtube.com/watch?v=UrluLxHLwQM