Hear rumbling from your water heater? Learn what causes it, when it's serious, and what to do before it becomes a costly repair.
That Rumbling Sound Is Your Water Heater Trying to Tell You Something
If you've started hearing a low, rolling rumble coming from your water heater — especially when it's heating up — don't ignore it. That sound isn't random, and it usually doesn't go away on its own. In most cases, it's a warning sign that something inside the tank needs attention. The good news is that catching it early can save you from a much bigger headache down the road.
The Most Common Cause: Sediment Buildup
In the majority of homes we service, rumbling water heaters come down to one thing — sediment. Over time, minerals like calcium and magnesium naturally settle at the bottom of your tank. This is especially common in areas with hard water. As the burner heats the water, it has to work through that layer of buildup, and the water essentially pops and bubbles beneath the sediment. That's the rumbling you're hearing.
Think of it like boiling water under a layer of crust. The heat finds its way through, but it's inefficient, noisy, and eventually damaging to the tank lining and heating element. A customer in Phoenix once called us saying her water heater sounded like it was "boiling rocks." Sure enough, when we flushed the tank, we pulled out nearly an inch of hardened mineral deposits. She hadn't had the unit serviced in over eight years.
Other Possible Causes Worth Checking
While sediment is the most likely culprit, it's not the only one. Here are a few other things that can create similar rumbling or knocking sounds:
Thermal expansion: Water expands as it heats up, and if the pressure relief valve or expansion tank isn't working correctly, you may hear knocking or rumbling as the pipes and tank adjust to pressure changes.
Loose components: Dip tubes, heating elements, and drain valve fittings can loosen over time. When hot water moves through the system, these parts can vibrate or knock against the tank walls.
Scale on the heating element: On electric water heaters specifically, scale buildup directly on the lower heating element can cause a similar rumbling or crackling noise as it heats up. This is one detail a lot of homeowners overlook when diagnosing the problem themselves.
How Serious Is It?
The rumbling itself isn't an emergency, but what it leads to can be. Sediment acts as an insulating layer, forcing your water heater to run longer and hotter to reach the target temperature. This puts strain on the tank, shortens its lifespan, and drives up your energy bill. Left unaddressed for years, it can cause the tank bottom to overheat and eventually crack or fail entirely.
In many homes, we see water heaters that are only 8 to 10 years old failing prematurely — not because of a manufacturing defect, but simply because they were never flushed. Most manufacturers recommend annual maintenance, but that's something most homeowners don't know about until there's a problem.
What You Should Do
Flush the Tank
The most effective first step is flushing the water heater to clear out sediment. This involves connecting a garden hose to the drain valve and letting the tank empty while cold water runs in. It sounds simple, but if your unit hasn't been flushed in several years, the sediment may have hardened and won't flush out easily. In that case, a professional should take a look before you accidentally damage the drain valve trying to force it open.
