What is the truth in The Tell-Tale Heart?

As the answer above suggests, it was the main character's own guilty conscience.  As we know, he is... a little mentally unstable.  The heartbeat is not from the old man's corpse, it is from his own delusions and insanity from the guilt that he made for himself by murdering the old man in cold blood.  Eventually, his guilt from killing the man and his fear of being caught boil over, and he confesses his crime in a dramatic breakdown.

the tell tale heart's truth is that the beating was his guilt, and without it, there is no tale. If the heart were not there, he would have suffered other ways, but there will be no story.

Explanation:

i'm not sure i'm so sorry i will see what i can do

c) eager anticipation.