STATUS: I figured since it was time for me to get a new toilet anyway, I went ahead and replaced EVERYTHING. New toilet, fillvalve, flapper, etc. I even went as far as to replace the Stop Cock with a 1/4 turn stop cock. Everything was checked for leaks and things looked fine. Much to my dismay, when I flushed the toilet (after putting some tissue in it), sure enough after all the tissue goes into the toilet, about a second later, a hear the same rumbling in the pipes. Any thoughts?