what sound made rainsford know he had new things to learn about fear

The sound of a baying pack of hounds alerted Rainsford to the fact that he had new things to learn about fear.

At daybreak Rainsford, lying near the swamp, was awakened by a sound that made him know that he had new things to learn about fear. It was a distant sound, faint and wavering, but he knew it. It was the baying of a pack of hounds.