Characters
Do you hear that Fezzik? That is the sound of ultimate suffering. My heart made that sound when the six-fingered man killed my father. The Man in Black makes it now.
— Inigo Montoya · The Princess Bride

Behind the line May contain spoilers.
Our reading
An editorial interpretation.
Inigo tells Fezzik the sound they hear is suffering at its most extreme. He links it to the sound his own heart made after his father's death. Now, he says, the Man in Black is the one making it.
Context reviewed