Could it be an earthquake?
Or perhaps it announced some volcanic explosion.

I stopped before that part of the rock where the noise was loudest; the surface was firm and level; but from time to time, blows and falling stones seemed to strike our ears.

I was uncertain what to do; curiosity prompted me to stay, but a sort of terror urged me to remove my child and myself.

However, Jack, always daring, was unwilling to go till he had discovered the cause of the phenomenon.

"If Francis were here," said he, "he would fancy it was the wicked gnomes, working underground, and he would be in a fine fright.