Quick Answer:
A sharp hammering or knocking sound that happens when your pool’s automatic refill runs is usually a water pressure shock — often called water hammer — or vibration from a valve or pump cycling. If the sound comes from piping or built-in systems you can’t reach, stop poking at equipment and have a plumber isolate the source safely and make repairs.
Why This Happens
When the pool autofill turns on and off, valves open and close quickly. That sudden change in flow can create a pressure surge that bangs against pipe walls or fittings. Other common causes:
- Fast-closing solenoid or float valves that produce a pressure spike.
- Loose pipe straps, hangers, or supports that let piping move and hit framing.
- Air trapped in the lines or a failing check valve causing pulses of noise.
- Nearby equipment like a pressure booster, softener, or pump that vibrates during its cycle.
Compare the timing and character of the noise to other appliances — if it only shows up during the refill, see the related topic Noise only during appliance cycles. If a pump is active when the sound happens, consider Noise when pressure booster pump runs.
Step-by-Step What to Do
1. Run the autofill cycle that triggers the noise
- Start the pool autofill so the system performs its normal cycle. Do this while someone else can listen or while you move around the house to locate the sound.
- If the autofill is intermittent, initiate several cycles to confirm consistency.
2. Listen for the exact phase (fill, drain, shutoff)
- Note whether the noise occurs right when water starts flowing, while it is flowing, or just when the valve shuts off. That timing points to different causes (valve opening, steady vibration, or sudden closure).
- Try to tell if the sound is a single thump, repeated knocking, or continuous vibration.
3. Touch nearby supply lines to feel vibration
- Carefully place your hand on exposed supply pipes, valves, and the pump housing while the cycle runs to feel for vibration. Use a cloth if pipes are hot or cold.
- Follow the piping route where you can access it (garage, equipment pad, basement) to find the strongest vibration point.
4. Compare with other appliance cycles
- Run other fixtures or appliances that use the same supply (washing machine, water heater, irrigation) and listen for similar noises. This helps decide whether it’s unique to the pool autofill or a general pressure issue.
- Note differences in loudness and timing between appliances.
5. Document what you find
- Write down when the noise happens, what you felt on the pipes, and any visible movement. This information saves time when you call a plumber.
- If safe, record a short audio clip of the sound for the technician.
What Not to Do
- Don’t disassemble appliances or bypass safety devices to chase sounds — that can create hazards and void warranties.
- Don’t remove pressure-relief or safety components from built-in systems to stop a noise temporarily.
- If the noise is tied to built-in systems like water softeners, booster pumps, or the pool’s automatic valves, have a plumber diagnose and repair it properly rather than trying to modify those systems yourself.
When to Call a Professional
- Call a plumber if the source is inside walls, ceilings, or under concrete where you can’t reach pipes safely.
- Also call when the noise is loud, recurring, or accompanied by leaks, reduced water flow, or pressure swings — these signs can indicate valve failure or potential pipe damage.
- A licensed plumber can safely isolate the line, test valves and pumps, secure loose piping, and install water-hammer arrestors or slow-close valves if needed.
Safety Notes
- Turn off electrical power to pumps and equipment before touching or working near wiring or motorized components.
- Never open pressurized plumbing; relieve pressure or shut off the main before attempting any work on valves or fittings.
- If you must get into attics, crawlspaces, or ceilings, use proper fall protection and avoid standing on unsupported surfaces.
- If you smell gas or see active leaks, evacuate and call emergency services or a technician immediately.
Common Homeowner Questions
- Is the hammering dangerous?
Usually it won’t cause immediate failure, but repeated water hammer can stress fittings and joints over time and should be checked. - Can I install a water hammer arrestor myself?
If you have plumbing experience and accessible piping, you can install one on a shutoff valve branch; for in-wall or complex systems, hire a plumber. - Will tightening pipe straps fix it?
Tightening loose straps can reduce vibration, but if the root cause is a valve or pump, securing pipes alone may not stop the noise permanently.
For more related articles, see the Noises Only When Appliances Run hub.