Why is my water heater making a popping or rumbling noise?

The noise is steam escaping through a layer of sediment. Here is the chain of events. Tap water carries dissolved minerals, mostly calcium and magnesium. When the water sits in a hot tank, those minerals separate out and sink. They settle on the bottom as a gritty, rock-like layer called scale.

On a gas water heater, the burner sits right under that floor. On an electric model, the lower heating element runs through the lower third of the tank, where sediment piles up. Either way, the heat hits water that is now trapped beneath or inside the scale. That pocket of water cannot circulate, so it overheats and flashes to steam.

Steam takes up far more space than water. The bubbles force their way up through the sediment bed and burst. Picture a covered pot of water at a hard boil with the lid rattling. That rattle is your tank. Pops and rumbling can come from heated water moving through sediment, but the sound alone does not show the depth or shape of the mineral layer.

The sound is a symptom, not the disease. The sediment is the real problem. The noise is just the first sign you can hear without opening anything up.

A new or worsening sound is a reason to check the tank, especially if hot-water output has changed. Volume and timing help describe the symptom, but they do not measure scale depth, energy cost or remaining tank life. Inspection can distinguish sediment from another heating or plumbing issue.

Phoenix tap water ranges from hard to very hard, which supplies minerals that can feed a sediment layer. Temperature, water chemistry and use also affect how quickly deposits form.

Water hardness is measured in grains per gallon. City of Phoenix water quality reports put local total hardness at roughly 10 to 17.6 grains per gallon (City of Phoenix, 2025 Water Quality Report). The USGS scale tells you what that means. The agency classifies water above 180 mg/L (about 10.5 grains per gallon) as "very hard," the top tier. The reported Phoenix range spans hard and very hard water; a test at your inlet shows where your home falls.

The reason is geography. Phoenix draws most of its supply from the Salt, Verde, and Colorado Rivers, which run through mineral-rich desert rock and pick up calcium and magnesium along the way. That mineral load is what lands in your tank.

Heat makes it worse. Calcium and magnesium come out of solution faster as water gets hotter, so the inside of a water heater is the single best place in your home for scale to form. The hotter you run the tank, the faster minerals drop out and settle. Every other hard-water headache you see around the house, the white film on shower glass, the crust on faucets, the spots on dishes, is the same mineral that is quietly stacking up on the floor of your tank.

Hardness can contribute to sediment even in a relatively new heater, but age and noise alone do not establish the cause. Check the model's maintenance history and rule out other sources before assuming scale or a defect.

Sediment does more than make noise. It can reduce gas heater efficiency and damage heating components. In a gas tank, the scale layer acts like insulation between the burner and the water. Electric storage heaters behave differently: the research reviewed by PNNL found rated efficiency was maintained until sediment buried the lower element.

The DOE report PNNL-22921, written by Pacific Northwest National Laboratory, reviews Battelle's accelerated tests. Gas storage efficiency fell from 70.4% to 67.4% over two equivalent years of service using water at about 444 mg/L hardness, high temperatures and heavy daily use. Those conditions were more severe than a typical household's, so the figures are a comparison rather than a prediction for your tank.

Reduced gas heater efficiency can mean more fuel is needed for the same hot-water use. Water heating is already about 13.1% of residential energy use, according to the DOE Building America Strategy Guideline. The effect on your bill depends on the heater, fuel price, scale level and hot-water use.

PNNL also describes how accumulated sediment can overheat gas tank material or bury and overheat an electric element. Noise alone does not measure the scale depth or show how many years the tank has left. Inspect the heater and follow its maintenance instructions rather than assuming a fixed loss of service life.

If sediment is the cause, draining and flushing can remove loose deposits and reduce noise. Follow the model's instructions, including shutting off the heat, cooling the water and completely refilling before restoring power. Hardened scale may need more than a basic flush.

A few cases need more than a basic flush. If the sediment has hardened into a solid layer, a simple drain may not lift it, and the tank needs a more thorough cleaning or, if it is old and corroded, replacement. A flush also will not help if the noise is coming from a different source, which we cover below.

To keep the noise from returning, two steps matter most in Phoenix:

• Follow the model's flushing schedule. Annual flushing is common guidance, but the maker's instructions and sediment found during service determine the interval. Flushing removes loose sediment; it does not always remove hardened scale. We cover timing in detail on our page on how often to flush a water heater in Phoenix.
• Consider treatment for measured hardness. A working ion-exchange softener reduces calcium and magnesium available to form scale. That can help gas and tankless heaters, but it does not guarantee a longer tank life or replace maintenance. Softened water can accelerate anode consumption in a steel storage tank. Review the heater's instructions and treatment needs before choosing equipment.

Have the anode rod inspected on the schedule in the heater's manual and replaced when its protection is used up. The rod corrodes on purpose to protect the steel tank. PNNL recommends closer attention when water is artificially softened because anode consumption can accelerate. Flushing does not restore a depleted rod.

One caution on flushing an older tank. If a tank has gone many years without a flush and is suddenly drained, the scale that was sealing tiny pinholes can break loose and expose a leak. That does not mean flushing caused the failure. It means the tank was already near the end and the sediment was hiding it. On a tank past the eight to twelve year mark that has never been serviced, it is worth weighing a flush against simply planning for a replacement.

Grit at the faucet along with the noise is a strong sign of heavy sediment, or of pieces of a failed dip tube. Our page on why there is sand or grit in your hot water shows how to check whether it is the city supply, the tank or the dip tube.

Not every water heater sound is scale, so it helps to know the other common ones. Most are harmless, but a couple are worth a closer look.

A light ticking or tapping, often right after the heater runs, is usually normal thermal expansion. Pipes and metal parts grow slightly as they heat and shrink as they cool, and the movement makes a soft tick. On its own it is nothing to worry about. The same expansion can make plastic drain and supply lines tick or knock elsewhere in the house, which why a drain makes a knocking sound covers. If the ticking comes with a heat-trap nipple or check valve, it can get a bit louder but still is not a fault.

A screeching or whistling sound can come from water moving through a restriction, such as a partly closed supply shutoff or a narrowed opening. The temperature and pressure relief valve is a safety device, not a supply valve to leave open. If it makes noise or discharges water, have the heater checked; do not force the relief valve open, cap it or block its discharge.

A loud rumble or boom on a gas unit at startup can sometimes mean delayed ignition, where gas builds up before it lights. That one is not a sediment issue and should be checked promptly. Popping and rumbling during normal heating can point to sediment, but other causes should be checked if the sound persists. When sediment is confirmed, removing it can quiet the tank. Choose any water treatment based on measured hardness and the heater's requirements.

For how scale affects tank and tankless heaters differently, see whether hard water will ruin a hot water heater, and whether a softener damages a water heater if you soften.

A hiss or sizzle rather than a pop can come from condensation or a small leak, see why a water heater is hissing.

The draining and anode-replacement guide explains the shutdown sequence and why the actual model decides the rod, tools and tightening method.