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:

overnment is strong when they have the ability to persuade those they govern to believe government has enough power to force them or should be forcing them to do government’s will.

when enough of the citizenry do not believe government can force them or should force them to do its bidding, government has lost or will lose its strength and power to coerce.

when enough of the citizenry do believe government can force or should force them to do its bidding, they become passive at the least or actively pro-government and repeat government’s own rhetoric to pass on what they have been persuaded by government’s propaganda.

a true majority isn’t always needed to govern or to stop government. many times all that is needed is a strong minority. if a determined 30% of the population rises up against government in a concerted effort to bring it down, this size minority usually will succeed as long as those actively opposing the minority are less in force than the minority. so if 41% of the population sits at home watching tv rooting for one side or the other, they don’t matter. it’s the active parties who make a difference.