My hot water heater is about ten years old and rattles like crazy when it is heating water. How do I stop the rattle and how do I know when to replace it?

See above the message on "Banging noise from
water heater"

draining the water heater may be your answer but it may not help at all. replace heater install water softener, and thermo expansion tank. when sediment builds up in the bottom of the tank and you try to drain. it only comes out where valve is causing uneven heating of tank and premature falure. thus soften the water into tank. and to relieve extreme pressure install a expansion tank on the cold water inlet side of the heater.