High Pressure
High water pressure is a very likely cause for water hammer in your washing machine. It may not have anything to do with the machine itself but with the rate at which the water enters your house. The pipes that carry the water may be too small for the water pressure that feeds the home. While higher water pressure may seem nice for showers or washing the car, this high pressure can be damaging to your washing machine and other plumbing throughout the house. The impact of the water, as water hammer occurs, can crack fittings, damage parts inside the washer and even rupture hoses that connect the appliance to the water supply.
Valves
When the washing machine needs water, the machine opens a water supply valve and lets in the desired amount before automatically closing the valve. This is similar to opening and closing a sink faucet. Some washing machine valves open and close too quickly or too slowly. This may be related to their design or to faulty valves. If the valve shuts off too fast, it could cause water hammer as the water slams into the quickly shut-off valve. If the valve closes too slowly, it could also cause noise and vibration in pipes.
Arrester
A water hammer arrester is a device that can be installed on the supply line of your appliance to prevent the sudden collision of water in the pipes as the washer shuts off. These devices provide a cushion of air to help slow down and ease the impact of the water. They are available in most plumbing stores and can be easily installed.
Other Similar Problems
Not all of the sounds you hear coming from the wall behind your washing machine are necessarily water hammer. They may be related to it but are not technically the same thing. The noise could be caused by air in the pipes. This is often the case when repairs have been made on the water lines in or around your home. Usually letting the faucets run in the house for a few minutes will clear it out and stop the noise from pipe vibrations. Another similar problem is loose pipes. The pipes behind your washer may not be secured very well. The water will cause them to move around a little; and if they are not secured, the pipes can bump into one another or against objects around them, like the wall. This can make a very loud noise that can be remedied by securing the pipes.