What is the grammatical name and function for the highlighted clause?
When we were about half-way through, I suddenly put my hand upon her arm, for I had heard in the silent frosty air a sound that brought my heart into my mouth—the tap-tapping of the blind man's stick upon the frozen road. It drew nearer and nearer while we sat holding our breath.
Treasure Island, ch. 4.
Is it adverbial clause or adjectival clause?