Nothing more was heard for several minutes; but then there came out of the depths faint knocks: tom-tap, tap-tom. They stopped, and when the echoes had died away, they were repeated: tap-tom, tom-tap, tap-tap, tom. They sounded disquietingly like signals of some sort; but after a while the knocking died away and was not heard again.

‘That was the sound of a hammer, or I have never heard one,’ said Gimli.

The Lord of the Rings, Book II, Chapter 4: “A Journey in the Dark”