The answer to 'why is my water heater making gurgling noise' is that a gurgling sound from your water heater is typically caused by sediment buildup at the bottom of the tank. As the water heats up, the sediment can cause air bubbles to form and release, creating a gurgling or rumbling noise.
Some other common causes of a gurgling water heater include a buildup of minerals, a problem with the dip tube, or a malfunctioning pressure relief valve. If the gurgling is accompanied by reduced hot water output or leaks, it's likely a more serious issue that requires professional attention.