The inertia of the moving water creates a pressure spike that can travel through the pipes. This is especially common in systems with fast-closing valves, like washing machines or dishwashers.

Is it bad, other than the noise?

Yes, water hammer can be more than just a noisy nuisance:

• Pipe Damage: The pressure caused by the shockwave can cause wear and tear on the pipes, joints, and fixtures over time. This can lead to leaks or even burst pipes if the issue isn’t addressed.
• Worn-out Seals: The repeated pressure surges can damage seals in faucets, valves, and appliances, leading to leaks.
• Increased Stress on the Plumbing System: The added pressure on the system can reduce the lifespan of your plumbing.

How to prevent or fix water hammer:

1/ Install Water Hammer Arrestors: These devices are designed to absorb the shockwave that occurs when the water is abruptly stopped. They can be installed at key points in the plumbing system (e.g., near washing machines, dishwashers, or other appliances with quick-acting valves).

2/ Secure the Pipes: If your pipes are loose or not properly anchored, they can vibrate more easily when a water hammer occurs. Securing them with pipe clamps or brackets can help reduce movement.

3/ Adjust Water Pressure: High water pressure can exacerbate water hammer. You can check your home’s water pressure with a pressure gauge and install a pressure-reducing valve if necessary to keep it within a safe range (typically 40-60 psi).

4/ Air Chambers: Some older plumbing systems have air chambers built into the lines. These can become waterlogged over time. You can drain the system to refill the air chambers, which act as shock absorbers.

If you’re dealing with water hammer, these steps can help reduce or eliminate the noise and prevent damage to your pipes.