Listen to the water heater in this video at the bottom of the page!
One of the things I look (listen) for when inspecting older gas water heaters (8 years or more) is noise when the water heater starts heating. I run the hot water until it’s gone, then wait for the water heater to start heating.
Sometimes I hear it. Sometimes I don’t.
The noise in the video below is from sediment on the bottom of the water heater tank rumbling around as the heat gets to the sediment.
Consider replacing the water heater for energy efficiency. Enough sediment has built up on the bottom of the tank that it’s bouncing around like rocks when the heater is operating. (Think of a pan of water on the stove with a layer of sand on the bottom of the pan.) This sediment has to be heated up each time the water heater fires up to heat water. This heater will cost more than normal to operate because of the sediment buildup.