What noise did they hear as stepped into the clearing in The Hobbit?

In The Hobbit, when Bilbo Baggins and the dwarves step into the clearing in the forest, they hear a loud, deep, booming laugh. It's the laugh of the troll, William, who is waiting for them.

The laugh is described as:

> "a deep, booming laugh, like the sound of thunder, that shook the ground beneath their feet."

This laugh is a clear sign of danger and foreshadows the encounter with the troll that follows.