Popping or rumbling from a tank-style water heater often happens when mineral sediment settles at the bottom and water heats beneath it. The noise may signal that the tank needs maintenance, but sound alone cannot confirm the cause. Loud new noises, poor hot-water performance, or moisture around the unit deserve an inspection.
A professional can check the heater and recommend an appropriate flush or repair for its age and condition. If you notice a leak, seek help promptly. Stanley offers water heater service.