Hard water leaves minerals behind wherever it is heated. The tank of a water heater is the spot where that happens most, because the burner or element keeps the water hot for hours at a time and the minerals settle to the bottom. Over time that layer of scale and sediment reduces how well the heater transfers heat, and you hear it.
The signs you can check yourself
Listen while the heater is running. A rumbling, popping, or crackling sound during a heating cycle is the most common sign. Notice whether hot water takes longer to arrive at the tap than it used to, or whether the water runs hot for a shorter time before turning lukewarm. A heater that works harder to reach the same temperature is often carrying sediment.
Hard water is not the only cause of these sounds. Sediment from the water supply, a failing element, or a heater set to an unusually high temperature can produce similar noises. That is why the sound alone is not enough to name the problem.
Confirm the water is the cause
Ask for the hardness reading of your water before you assume the tank is the issue. If the reading is high and the softener is not working as intended, the heater is getting hard water and building scale. If the softener is working and the water tests soft, look at the heater itself and at other possible sources of sediment.
Check the owner's manual for the heater. Most manuals list a flushing or inspection interval and describe how to drain sediment safely. Follow those steps, and turn off power or gas to the unit before you open any valve.
What to ask a plumber or technician
Ask whether the heater can be flushed, whether the sediment is affecting the element or burner, and whether replacing the heater makes more sense than cleaning it. Ask whether a softener adjustment would reduce the scale going forward. A clear answer will name the cause and the fix. A vague answer that only says the tank is old is not enough to decide on.