On Wed, 25 Aug 2010 08:32:54 -0700 (PDT),
wrote:

I've turned off the toilet and yes, the water level is half of what it
should be in a few hours.

Lift the flapper and feel around the flapper seat checking for damage
or deformities. Also check that the flapper is installed correctly
and seats properly.

I had a toilet "chatter" in the middle of the night. Removed the tank
lid, wiped the cooties (slim) off the seat and flapper and the
flapper has been seated ever since.

If the tank was broken (cracked) or the tank bolts failed water would
be on the floor.