This is the most common noise by far. Minerals settle out of hard water and form a crust on the bottom of the tank. The burner heats through that crust and water trapped underneath flashes to steam, which escapes with a pop. Flushing the tank clears it and the noise usually stops.
The crust insulates the water from the burner, so the heater burns more gas for the same hot water. It also overheats the tank bottom, which shortens the life of the glass lining. A yearly flush is genuinely worth the hour it takes across most of California's hard water regions.
A high-pitched noise usually means water is being forced through a partly closed valve. Check that the cold inlet valve is fully open. If the noise comes from the temperature and pressure relief valve, stop and call a plumber, because a T and P valve venting is a safety issue, not a nuisance.
Metal pipes expand as hot water moves through them and tick against framing or strapping. It is harmless, though a heat trap nipple or a loose strap can make it louder than it needs to be.
This is not the heater at all. It is pressure surging when a valve closes quickly, and it is fixed with an arrestor or by correcting excessive house pressure. Left alone it stresses every joint in the system.
If a tank rumbles heavily, has rusty hot water, and is more than about ten years old, flushing buys time but the tank is near the end. Getting a replacement quote before it fails is much cheaper than an emergency swap after it floods a garage.
A real plumber, a flat-rate quote, any hour of the day.