Our unit was installed in 08, it has all original parts, and has worked fine. However, during the last few days we've noticed it is slow in filling and makes a longish "burping" sound, and then a kind of hiss just as it shuts off. Obviously, these are highly subjective terms, but it's the best I can do.
I've gone through most of the posts, and can't find anything that seems to fit. The tank fills to about an inch below the top of the flush valve. The valve itself feels properly seated, though I haven't checked for cracks. There is no sound of leaking water, however. I've pressed down on the flapper to try to make sure there's a good seal while the fill is going on. There's slack in the chain above the "float," but it is taut below.
As for the sound, I saw that you used the term "water hammer" to describe another member's problem. Regarding pressure on the water heater, etc., we did drain the water heater and shut off the water supply to the house for three months during the winter.. Activated everything six weeks ago.