Quick Answer:

A banging or knocking noise while filling the tub is usually caused by air in the lines, fast-moving water striking loose pipe supports, or a water hammer effect when valves close. Start with simple checks: purge the lines of air, fill the tub slowly, test hot and cold separately, and see if other fixtures do the same. If the noise is violent or you see pipe movement or leaks, call a plumber.

Why This Happens

Common causes of banging or knocking when the tub fills:

• Air trapped in the pipes after a recent shutoff or repair. If air pockets move with the flow they can cause irregular pressure pulses.
• Water hammer—sudden pressure spikes when a valve closes quickly. This creates a loud banging and can stress fittings.
• Loose or unsupported pipes that vibrate or hit framing as flow changes.
• Valve or cartridge problems in the faucet that allow quick on/off or intermittent flow.

If the problem started after a water shutoff or system work, check guidance on pipes banging after turning the water back on for that specific scenario.

Step-by-Step What to Do

1. Purge air from the lines

Air in the pipes is a common cause. To purge:

• Open the tub faucet full and let water run for 1–2 minutes. If your home has a second-story fixture, run it too to help move air through the system.
• Run both hot and cold taps separately to clear air on each line.
• If you recently had a shutoff, repeat this at other fixtures to push air out of the system. For more detailed steps see how to clear air from pipes.

2. Fill the tub slowly

Turn the faucet on to a slower flow. Slower flow reduces pressure surges and gives air pockets a chance to escape quietly. If the noise drops or stops at low flow, it points to pressure or valve-related causes rather than an immediate plumbing failure.

3. Test hot vs cold

Open only the hot side, note any noise, then close and test only the cold side. If the noise occurs only on one side, the issue is likely in that supply line, faucet cartridge, or the water heater (for the hot side).

4. Check other fixtures

Run sinks, showers, and other taps while the tub is filling. If the same knocking happens elsewhere, the problem is likely in the main supply or a shared section of pipe. If it is only the tub, focus on the tub shutoff, faucet cartridge, and the short section of piping to the tub.

5. Identify water hammer symptoms

Water hammer signs to watch for:

• Very loud, sudden bangs when a valve closes.
• Movement or knocking of exposed pipes.
• Repeated pounding when multiple fixtures cycle.

Water hammer usually needs a targeted fix such as arrestors, securing loose pipes, or servicing quick-closing valves. If the banging is rapid and violent, do not ignore it—see the When to Call a Professional section below.

What Not to Do

• Do not keep snapping the faucet on and off. Rapid cycling can make damage worse and won’t diagnose the root cause.
• Do not ignore violent hammering. Persistent loud bangs can loosen fittings or cause leaks.
• Do not clamp pipes without diagnosing. Clamping or forcing pipes can damage them or hide the real problem.

When to Call a Professional

Call a licensed plumber if any of the following apply:

• The noise is loud and continuous or accompanied by visible pipe movement.
• You notice leaks, drips, or damaged fittings after the banging starts.
• Simple fixes (purging air, slowing flow, testing hot vs cold) don’t stop the noise.
• Water hammer symptoms persist; a plumber can install arrestors or secure piping safely.

A pro can diagnose pressure-related issues, replace faulty cartridges or valves, and secure pipes without risking damage.

Safety Notes

• If you need to shut off the main water supply, know where the shutoff valve is and shut it off slowly to avoid pressure shocks.
• Do not attempt to modify or clamp hidden pipes yourself. Cutting, soldering, or altering pressurized pipes can be dangerous and should be left to professionals.
• Turn off the water heater before doing work on hot water lines if you are unsure how to proceed; if in doubt, call a plumber.

Common Homeowner Questions

• Is banging dangerous?
  Short, mild bangs are often harmless; persistent violent banging can damage fittings and should be checked.
• Can I fix this myself?
  Yes for simple cases—purging air, filling slowly, and testing hot vs cold can often help. Call a pro if it continues.
• Could the water heater be the cause?
  If the noise happens only on the hot side, the heater or trapped air in its lines could be involved; test and call a technician if needed.