A water heater making knocking noise can point to more than one cause, and the biggest clue is timing. A single bang right when water flow stops somewhere in the house often points to water hammer — a plumbing shockwave, not a tank problem. A knock or pop that repeats specifically during the heating cycle points somewhere else: sediment buildup, or another issue inside the heater itself. This guide walks through how to tell the two apart and what to do next.
What the Timing Tells You
Instead of assuming a single cause, start by noticing exactly when the knock happens:
- Right when a faucet, washing machine, or dishwasher shuts off: this timing points toward water hammer, a pressure shockwave in the plumbing — not the water heater itself.
- Only during the heating cycle, while the unit is actively heating water: this timing points toward something happening inside the heater, most commonly sediment buildup at the bottom of the tank. Our guides to a water heater making popping noise and a rumbling water heater cover how sediment produces these sounds in more detail.
- A light, rhythmic tick, unrelated to either pattern above: this is usually normal thermal expansion of metal parts and doesn't need action.
If what you're hearing is closer to a continuous rumble than a distinct knock, that points more specifically to heavier sediment buildup — see our rumbling water heater guide.
Matching the Pattern to a Likely Cause
| Pattern | Possible cause | Recommended next step |
|---|---|---|
| Single bang right when a faucet or appliance shuts off | Water hammer (plumbing shockwave) | Notice which fixture triggers it; call a plumber if it persists |
| Knocking or popping that repeats during the heating cycle | Sediment buildup, or another issue inside the heater | Professional inspection if it persists |
| Metallic clanking elsewhere in the house, not just near the heater | A plumbing issue unrelated to the water heater | Have a plumber inspect the piping |
What a Water Hammer Arrestor Does
If the pattern points to water hammer, a plumber may recommend a water hammer arrestor — a small plumbing component that absorbs the pressure shockwave using an internal air cushion, installed on the affected supply line. It's generally an inexpensive plumbing component, though cost varies by home, location, and how many fixtures are affected. A plumber can determine the correct location and device for your situation — homes with the problem at more than one fixture may need more than one arrestor.
What to Pay Attention To
Rather than testing anything directly, just notice the pattern: does the knock line up with a washing machine, dishwasher, or faucet shutting off? Does it happen consistently at the same fixture? That observation alone gives a plumber a strong head start — there's no need to try to reproduce the noise yourself.
If It's Coming From Inside the Unit
Knocking or popping that's clearly tied to the heating cycle points to something happening inside the heater itself, not the plumbing. Don't open any access panel or attempt to inspect internal components yourself — have a licensed plumber diagnose it, especially if the sound persists or gets louder.
When To Call a Plumber
- The knock happens specifically during the heating cycle, not tied to a fixture shutting off.
- The sound is getting louder or more frequent over time.
- The pattern doesn't clearly match either water hammer or the heating cycle.
- You're not comfortable identifying which fixture — or which pattern — you're dealing with.
Bottom Line
A water heater making knocking noise isn't always the same problem — the timing tells you the most. A bang tied to a fixture shutting off points to the plumbing, where a water hammer arrestor is the typical fix. A knock tied to the heating cycle points inside the heater, often to sediment, and is worth a professional look if it persists.