This passage is showing how the sound made the character more aware of the situation.
He was not aware of what he was seeing until he heard a muffled grating sound. Even though the sound was muffled, after that the paid attention on the object, the black mark, that was moving, and then the sound lost its significance.
If there would not be the sound, the character would not pay attention to the object and that is why it is showing how sound and sensory information are related.
