The answer to 'why is my water heater making loud noise' is that it is likely due to sediment buildup or mineral deposits inside the water heater tank. This can cause the water to boil and create a rumbling or popping sound as the bubbles rise to the surface.
Other potential causes include a malfunctioning thermostat, a problem with the heating element, or high water pressure. If the noise is accompanied by leaks or other issues, it's best to call a professional HVAC technician to inspect and repair the water heater.