Ticking or tapping when hot water runs is usually thermal expansion. Hot water makes the pipe grow slightly, and it rubs or clicks against a strap, joist or framing hole as it slides. The same tick returns as the pipe cools. It is usually harmless, and cushioning or re-supporting the pipe stops it.
What causes the ticking
Every piping material grows when it heats up and shrinks when it cools. The Copper Development Association says in its Copper Tube Handbook: "Copper tube, like all piping materials, expands and contracts with temperature changes." The amount is small but real. The handbook works an example: "the expansion of each 100 feet of length of any size tube heated from room temperature (70°F) to 170°F (a 100°F rise) is 1.128 inches."
A house doesn't have 100 feet of straight pipe in one place, but a hot line that runs the length of a hallway or up through a wall still moves a fraction of an inch each time it heats. If that pipe sits snug in a hole through a stud, against a strap or against another pipe, it can't slide quietly. It sticks, then slips, and each slip is a tick or a tap. Run a hot shower and you may hear a series of clicks for the first minute. Turn the water off and you hear it again as the pipe cools and pulls back.
That pattern is the giveaway. Ticking from expansion:
- Shows up as the hot water arrives and the pipe warms, not the instant a valve opens.
- Happens on the hot line and usually not the cold one.
- Repeats at irregular, slow intervals rather than a fast steady beat.
- Comes back as the pipe cools after the water stops.
The plumbing code expects this. The Phoenix plumbing code, based on the 2024 International Plumbing Code, says in section 305.2: "Where necessary, provisions shall be made to protect piping from damage resulting from expansion, contraction and structural settlement." When those provisions are missing or tight, you hear it.
Where the noise usually comes from
Four places account for most of it, and the first is the one to check first.
Pipes rubbing framing or straps. This is the most likely source. Metal straps, notched studs, tight drilled holes and pipes touching each other all catch a moving pipe. If you want the spacing rules for straps and hangers, see pipe hanger and support spacing.
Heat traps at the water heater. Some tank water heaters ship with heat trap fittings on the hot outlet and cold inlet to save energy. Rheem's installation manual describes them: "For increased energy efficiency, some water heaters have been supplied with factory installed 3/4 in. NPT heat trap fittings in the hot outlet line and cold water inlet line." Rheem's noise guide lists "Ticking or clicking: Often caused by heat traps or pipe expansion as temperatures change" among the sounds that are part of normal operation, and adds that quiet, brief noises at regular intervals are generally nothing to worry about.
Supports that are too tight or too far apart. The plumbing code sets maximum spacing for hangers by pipe material. Its table lists 32 inches of horizontal spacing for PEX pipe 1 inch and smaller, and 6 feet for copper tubing 1-1/4 inches and smaller. A pipe hung farther apart than that can flex and knock, and a pipe clamped hard can't slide at all.
A long or straight run with no room to move. The Copper Tube Handbook says a long line "tends to buckle or bend when it expands unless compensation is built into the system," and adds that "Severe stresses on the joints may also occur." It names the fixes used in larger systems: offsets, "U" bends and coil loops. In a house, the same idea shows up as a pipe that has a bend or a little slack instead of a rigid straight line between two fixed points.
Pipes in tight spaces. Runs through joist bays and tight framing have less room to slide, so that's where you tend to notice a tick.
Is it ticking or water hammer?
They feel different, and it matters because one is harmless and one can harm the plumbing.
| Thermal ticking | Water hammer | |
|---|---|---|
| Sound | Light ticks, taps, clicks | A bang or thud, sometimes a shudder |
| When | While hot water runs and while the pipe cools | The moment a valve or appliance shuts off |
| Both lines? | Hot line, mostly | Hot or cold |
| Cause | Pipe growth against a support | A pressure surge from stopped water |
If you hear a bang when a faucet closes, that's a different problem with different fixes, and it's covered in why pipes bang when you shut off the water. If the sound is a hum or a whistle while water runs, see noisy toilet fill valve whistling for one common cause, or water pressure that suddenly got too high.
How to quiet it
Here's the plan, from easiest to most involved.
- 1Find the spot. Run hot water at a faucet, then put your ear or a cardboard tube near the wall and follow the noise. Basements, crawl spaces and garages let you see the pipe directly.
- 2Cushion contact points. Where an exposed pipe touches a joist, strap or another pipe, foam pipe insulation, a rubber sleeve or a strap made for the pipe often ends the tick.
- 3Loosen or replace tight hangers. A strap that clamps the pipe hard stops it from sliding. A hanger sized for the pipe with a soft liner lets it move.
- 4Add support where the pipe sags. A pipe that hangs unsupported can flex and knock. Match the spacing to the pipe material.
- 5Leave hidden pipes alone. Cutting into a wall for a tick is rarely worth it. Pipes inside finished walls that tick lightly are common, and living with them is a reasonable choice.
Mind what you tighten. The code allows for movement in systems designed to expand: the hanger table notes that for "piping systems designed to provide for expansion/contraction," support intervals "shall conform to the engineered design." A rigid clamp on a run that's meant to slide can turn a small tick into a joint that's stressed.
When ticking is worth a call
A tick alone is fine. Get it checked when:
- The noise is a bang, a shudder or a vibration and not a tick.
- You see a wet spot, stain or drip at a joint near the sound. Movement puts stress on joints, so a damp fitting near a ticking pipe is worth a look.
- The noise is new after a plumbing repair or a new water heater.
- The sound seems to come from the water heater itself along with hissing, popping or dripping. Rheem lists hissing or sizzling sounds among the warning signs that call for a professional look.
Our licensed and insured plumbers can trace a noise to its source. The estimate is free, and you see the price up front before any work starts. If a joint is leaking or you smell gas near a water heater, that's an emergency and we're available 24/7.
If what you're hearing is running water and not ticking, see why you hear running water in your walls. Most of the time, a ticking pipe is just a pipe doing what pipes do when they warm up. Find where it's rubbing, give it a cushion, and it goes quiet.