About three weeks ago my, water heater died and we had it replaced.
When they did so, they had to add a tank on the top of the water
heater that was not their before (http://www.lowes.com/lowes/lkn?
action=productDetail&productId=160680-48324-LET-2&lpage=none not the
exact one but its basically the same thing). Its now required in my
city due to some water meter changes the city made. Anyway, as soon as
the guys (yes I hired someone) finished the toilet started to hiss. I
don't think that an abnormal amount of water is going from the tank to
the bowl so its not a constant refilling issue. If I close the shut
off value the noise stops, and when I open the value the noise starts
again. If I leave the valve open for a few days the noise gets louder
(it gets to the point of annoying and then I close the valve). Here is
another strange point. I was changing a hot water shut off value in
the kitchen, and I noticed when I close the hot water valve coming
from the water heater the noise in the toilet jumped.

Anyway, whats going on to make the hissing noise?

Should I just get one of those replacement kits at Home Depot or Lowes
and see what happens (noticed they are less than $15).

John