Gibbons often accompany their morning song bouts by spectacular locomotor displays that may include branch shaking and branch braking. These displays typically occur at the climax of the great-call, the most conspicuous and stereotyped song phrase of the female. Here I report on a captive female white-handed gibbon slamming the sliding door of her wooden sleeping box during the climax of her great-call. This special addition to her display produced a single, loud bang which acoustically accentuated the climax of the female’s great-call, made her great-call sound unique, and possibly enhanced the call’s effect on potential receivers (presumably female conspecifics). The female’s use of a door to modify her duet contributions represents a novel behavioural variant, and one of the few cases of tool use in gibbons or small apes. Furthermore, behavioural innovations like this one may have played a role in the evolution of human music.