Maybe this will be useful. This is from Frank's book: "I've seen Michael experiment with these kinds of sounds himself. He would record the rumble of a door closer's spring and then play with it, mixing it with a snare drum to create a completely unique and original sound. One day Michael put the microphone in a bag with rocks and toys and bits of metal, strapped it to the outside of a DAT machine wrapped in cushioning cushions, and threw the whole thing down the stairs, then took the sounds from the bag, laid them out on the console, and mixed them in harmony.On the Invincible album, these unusual effects are heard in the songs "Invincible", "Heartbreaker", "Unbreakable" and "Threatened""