I think it had whispered to him things about himself which he did not know, things of which he had no conception till he took counsel with this great solitude - and the whisper had proved irresistibly fascinating. It echoed loudly within him because he was hollow at the core

The lines stated in question are taken from Heart of Darkness written by Jospeh Conrad.

The context being told is regarding Mr Kurtz, a person who has been shown as a character with abilities of an idealistic man.

The paragraph is being described the manager of Kurtz. He thought that something had been told to Kurtz, things about himself that he didn't know until he was in isolation. That whisper echoed loudly inside him because he was hollow