Short answer
Rumbling or popping means sediment from hard water has built up in the bottom of the tank. Water trapped under the sediment boils in bursts as the burner or element heats it. Flushing the tank can help if the heater isn't too old. A loud, old tank is often close to replacement.
What's happening inside
Every time the tank heats Valley water, calcium carbonate drops out and settles. Over years it forms a layer that traps water against the heat source. That water flashes to steam, and you hear it as pops, cracks and rumbles.
Why it's more than a noise
- Sediment insulates the burner from the water, which wastes energy
- It overheats the tank bottom and shortens the tank's life
- In electric heaters it buries the lower element and burns it out
Flush or replace?
If the heater is under about 8 years old, a professional flush can quiet it and extend its life. If it's older, or the sediment has hardened, flushing may not help much and the drain valve may not reseal. Replacing it, and adding a softener, is often the better spend.
When to call a plumber
- There's rust-colored water or any leak at the tank.
- The tank is over 10 years old and noisy.
- Hot water runs out much faster than it used to.
What the plumber does: see water heater flush & descaling.
Common questions
Is a rumbling water heater dangerous?
The noise itself usually isn't. A leak, a dripping relief valve or a gas smell is, so shut the unit off and call.
Editorial note: this guide is general information for Phoenix-area homeowners, not a diagnosis. Conditions vary by home, so a licensed plumber should confirm the cause before any repair.