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.
memory
Don't confuse with 咯 which can also be read kǎ (cough up) and luò (a sentence-final particle). In 咯吱 it's always gē.
