When my water heater is heating, it makes a rumbling sound.  What does this indicate?  Gary in Prescott

Gary, the sound you are hearing is calcium build up in the tank, when the burner fires up and begins to heat the water.  This causes air pockets to form in the calcium, emitting little explosions of air.  Once you hear this in your water heater, it is time for replacement.  The rule of thumb give our location/altitude, is an 8-12 year life expectancy for a water heater.