The answer to 'why is my water heater making loud noise' is that your water heater may be experiencing a buildup of sediment or mineral deposits. This can cause the water heater to make rumbling, banging, or popping noises as the sediment heats up and expands. Over time, these deposits can also reduce the efficiency of your water heater and lead to premature failure.
Some other common causes of noisy water heaters include a faulty dip tube, a problem with the heating element, or issues with the pressure relief valve. If the noise is accompanied by leaks or other problems, it's best to have a professional HVAC technician inspect the water heater to diagnose and resolve the issue.