Answer: The narrator clearly cannot be trusted because he's a little crazy. He says he is not crazy, he denies it, but it's true and because of this, he is most likely imagining things, especially like the man's heartbeat every time he hears it. And because he is definitely imagining the man's heartbeat, we have no idea what other things he could be imagining.
The character's tone is written to be very nervous and on high alert, similar to a person ingesting a lot of caffeine... lol. He is not in the right state of mind and there are many pieces of evidence in the text that support this claim.
Explanation: I hope this helped and make sure to look back in the text and quote it or give examples as the question asks!