I may be a complete nub but the Cat has a Brake to shift solenoid that alot of folks say causes this sound your hearing... also if the last shop that changed your pads in the rear failed to re-install one of your retaining clips then you might hear a Pop or Clank when Braking.. but id check your parking brake assembly just to be sure theres nothing loose there.. also sometimes the brake line assembly will pull away from the bdy clips and when pressure is applied to the lines they can Flex causing the brake lines to Slap the undercarraige and may be causing this noise.. thats where i'd start and keep us posted, we'll help ya track that Gremlin Down..