You can try (but it probably won't help) flushing it out a little. Sometimes, you can remove the drain valve and poke a stiff wire or rod in there and crack out some. If it is electric, you can try removing the elements (they're not expensive). Too much poking/proding/banging and you make start a leak. It sounds like you have mineral deposits. These can act like insulators and can have pockets in them. WHen the heater turns on, they can cause localized bursts of steam as things get hot. These can sound like mini explosions and really rattle things. It could last a long time yet, or it could fail soon. You might want to budge for a replacement. The plumbers here recommend Bradford-White or Rheem; avoid the Whirlpool sold at Lowes like the plague. The GE at HD is made by Rheem, and is a decent unit.