Sounds like the baffles in the flue inside the tank are moving around.

They may make noise during heating, cooling, both or neither.

Have you checked your static water pressure? Should be less than 80psi

If your plumbing has a PRV (pressure reducing valve) at the water supply you should also have a thermal expansion tank near the WH and that tank should be checked for proper operation otherwise your plumbing, appliances, fixtures, and WH will see dangerously high water pressure.

click here to learn about thermal expansion

Could be the baffles or could be sediment accumulating at the bottom of the tank.

If you have hard water it could also be calcium (hardness) that has precipitated out of the water accumulating on the bottom of the tank.

Have you ever had your water tested? How long was the old WH in service? Was the old WH replaced because it leaked or another reason?

The old water heater was 8 years old and there was a problem with the burner assembly.I know my water psi is about 76. The noise is definitely when the burner is off and has been off for 5 0r 10 minutes. So I don't think it is sediment but like suggested must be a baffle or something contracting when cooling down. I am getting used to it know.