There are many synonyms for the phrase "hear his heart beating." Some of these include "listen to his heartbeat," "feel his pulse," "detect his cardiac rhythm," and "perceive his thoracic pulsations." Other synonyms may include "sense his chest vibrations," or "pick up on his cardiac cadence." All of these phrases describe the act of hearing or feeling someone's heart beating, and they can be used interchangeably depending on the context. Whether you're describing a romantic moment or a medical emergency, these synonyms offer a range of options for expressing the same sentiment.