I have (had) the same model and it is probably due to calcium build up at the bottom. Water heaters need flushed occasionally (I think every other year). Anyway, NOTE: I had mine flushed one time about 4 years ago. Yesterday, the bottom went out and flooded part of the house. Mine was installed September 2001 - I see yours was installed in 2002. I would look into replacing it before it goes.