The main reason for creaking noise in water heating systems is the thermal expansion, so when the pressure inside the tank increases the tank stretches, and when the pressure is relieved the metal tank returns to its natural shape.
A pipe could also make the creaking noise when the heater is recovering and all valves are closed. These valves include the check valves, which are used to prevent mixing the potable residential water with municipality water system and pressure reducing valves that are used to conserve water, also a water softener that acts as the back-flow valve.
If you are not sure about thermal expansion and does the pressure increases over the recommended maximum, follow these steps to test it (it applies to the closed system only):
Put the thermostat setting to the lowest position.
Install the water pressure gauge on the drain valve, which you should open for testing.
Open hot water tap and drain approximately 20% of the tank volume.
Shut off the drain valve. Turn off all the fixtures in your home.
Allow water heater to cycle by increasing the temperature on the thermostat.
As the temperature of the water inside the heater’s tank increases, the pressure will increase too, rapidly. If the pressure exceeds the maximum setting on the PRV or TPR valves, the valve should open and release water. Also, valves will close once the pressure drops below its maximum setting.
In order to avoid the high water pressure and protect the heating system from creaking noise and deformation, both the PRV and thermal expansion tank should be installed. The benefits of having the pressure relief valve installed are to save water, prolong the life of water flow valves and allow the selection of economically priced expansion tank.
The most used expansion tank is the tank that has two chambers and one flexible membrane in between. One chamber is filled with water, the other with air, so when there is water expansion, there is always room in the air chamber to compensate for the increase. When this device is installed, and the pressure starts to climb, there will only be a slight pressure increase to a point where it will stay and keep the steady pressure.