'It's just bubbles of hot water "exploding" into steam on the bottom of
the heater shell where the flame hits.'

What youve described , is, condensation that forms when you run the
water heater under high demand (cool water inside the tank being heated)
This makes more of a 'sisssssssss- ing ' sound than a pop ; if the
popping noise occurs ALL the time the water heater is firing (even
during a normal cycle of low demand...), then, its sediment in the tank
instead of condensation on the outside of the tank. As a safety
precaution, id replace a gas water heater at 15 years maximum ..and less
, if its located in an area that will cause major damage when it
eventually leaks..