咯吱 is a sound word, almost always doubled to
咯吱咯吱 in writing — covers the creak of old floorboards, joints popping, doors groaning, and the crunch of crisp food. The mouth-radical
口 on both characters is the standard marker for onomatopoeia. Xinhua glosses it as
扁担压得咯吱咯吱地直响 — 'a carrying-pole creaking under the weight'.
