Final answer:
Harry heard a low, rumbling roar outside the shack, similar to a sea shell. The sound is called 'Oldring's knell' and is believed to foreshadow Oldring's death. It is a hollow, unearthly howl made up of a thousand piercing cries.
Step-by-step explanation:
The loud sound Harry heard outside the shack is described as a low, rumbling roar that is similar to the roar in a sea shell. The sound is referred to as 'Oldring's knell' by the rustlers, who believe it bodes Oldring's death. It is a hollow, unearthly howl made up of a thousand piercing cries.
Therefore, Harry heard a low, rumbling roar outside the shack, similar to a sea-shell. The sound is called 'Oldring's knell' and is believed to foreshadow Oldring's death.