I've got a gas hot water heater that rattles when hot water is used from any/all spigots in the house. It only rattles when the hot water is running though it. It sounds like something bouncing around in the main body of the tank, not up near the top, more like in the middle. It's a Bradford-White unit, installed in 1995. We've done nothing to it since buying it in 2002. We don't have any water issues around here (no deposits on anything). The rattle seems to have developed within the past 6 months.
This is a simple one. Bradford white and several others have, for lack of a better term, flo-cheks in them, either in the HOT outlet side or COLD inlet side or both. They did this to try to prevent HOT water from backing up in the COLD side of the tank. When they are new there is no problem, but as the tank ages the plastic cage for the check ball hardens from heat and abrasion and takes on a fatter inside dimension that allows the metal or glass marble-size ball to rattle around when the HOT is turned on. Most people choose to ignore it as long as it is not service affecting, but if it gets on your last nerve either the ball or the whole ball-chek assembly can be removed.
If you have die-electrics for connectors on your tank turn off your water supply to the tank and open the bottom tank spigot. Let it drain for 15 minutes or so and loosen the big nut on the die-electrics on both sides of the tank. when you have them both completely loose and there is no flood unscrew the bottom half of the fitting out of the tank. Do the HOT side first. On some tanks the HOT cage is right there looking at you. It's pink in color. The COLD side is blue so you can tell that you are not working on a Model "A". Stick a pair of long nose pliers in around the ball and turn them counterclockwise like a screwdriver to unscrew the cage from the tank. On some tanks the diptube for the tank has the ball assembly in it, but it can be pried or popped out with a medium flat blade screwdriver. You will have to rewrap the die-electric threads with some teflon tape before you put it back in or you will never stop the water. Finish one side before you start the other.