Short answer: Rheem explains that popping and rumbling come from sediment at the bottom of the tank, which annual flushing addresses. Quiet ticking (pipes expanding) and humming (elements on electric units) are usually normal. Hissing, sizzling or crackling can mean water is leaking onto a burner or element: turn off the power or gas and call a pro. Banging when a faucet closes is usually water hammer in the pipes, not the heater.
Quick diagnosis
| What you notice | Most likely cause | What to check | Call a pro? |
|---|---|---|---|
| Popping or rumbling while heating | Sediment on the bottom of the tank | Flush the tank | If it continues after flushing |
| Ticking or clicking | Pipes and heat traps expanding | Nothing if quiet and brief | No |
| Steady humming (electric) | Heating elements running | Nothing | No |
| Hissing, sizzling or crackling | Water dripping on a hot burner or element | Turn off power or gas | Yes |
| Screeching or whistling | Partly closed valve, pressure issue or T&P valve | Make sure valves are fully open | If it persists |
| Bang when a faucet shuts | Water hammer in the pipes | See pipes banging | Sometimes |
Sediment: the most common cause
Minerals in water settle at the bottom of tank heaters. Water trapped under the sediment boils and pops. Hard water makes it worse. Rheem recommends flushing annually; the noise may not go away fully on an old tank with heavy buildup.
Noises that need attention now
- Hissing or sizzling with water around the base or burner.
- Crackling with signs of leaking.
- Screeching that does not stop after checking valves.
- Any noise plus water leaking from the tank or the relief valve (see water heater leaking).
Flushing basics
Turn off the power or set gas to pilot, attach a hose to the drain valve, run it to a drain, and open the valve until the water runs clear. Follow your heater’s manual; hot water can scald, so let it cool first or run it carefully.
When to call a professional
- Hissing, sizzling or crackling sounds, or any leak.
- Rumbling that continues after flushing, especially on an older tank.
- The relief valve releases water or the water gets dangerously hot.
What it usually costs
HomeGuide (2026) lists flushing a tank water heater at about $75 to $250 and T&P relief valve replacement at about $50 to $300. Most repairs run $150 to $700.
Frequently asked questions
Is a popping water heater dangerous?
Popping from sediment is not an emergency, but it lowers efficiency and can shorten the tank’s life. Flush it.
Why does my water heater make a knocking noise?
Usually water hammer in the plumbing when valves close quickly, not the heater itself.
Related problems
- Water heater leaking
- No hot water
- Pipes banging
- Water heater keeps tripping breaker
- Hot water smells like rotten eggs
Sources
This guide is general information, not a substitute for an inspection by a licensed professional. See our disclaimer.