In this vintage interview, sound designer Ben Burtt discusses creating the sound of AT-AT walkers in The Empire Strikes Back.
Walkers were fun to work on for Burtt because "they were big mechanical beasts," and were a nice break from working on voices. Burtt felt that the challenge was to give them, through sound, a sense of mass and weight -- in reality, AT-ATs were just miniatures animated on a small stage.
The sound of the AT-ATs moving came from a metal stamping machine, something akin to a "big chopping block" for steel, recorded in a factory. Burtt took the sounds and made them rhythmic to convey walking. For its knee joints, Burtt took a dumpster door and recorded it opening and closing.