The main reason for the creaking noise in water heating systems is thermal expansion. The expansion happens when the pressure inside the tank increases, so 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 used to prevent mixing the potable residential water with the municipality water system and pressure reducing valves used to conserve water, and a water softener that acts as the back-flow valve.
If you are not sure about the thermal expansion and does the pressure increases over the recommended maximum, follow these steps to test it (it applies to the closed system only):
In order to avoid the high water pressure and protect the heating system from the creaking noise and deformation, both the PRV and thermal expansion tank should be installed. The benefits of having the pressure relief valve installed are saving water, prolonging the life of water flow valves and allowing 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 there is always room in the air chamber to compensate for the increase when there is water expansion. 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.