吱吱 (zhī) is an onomatopoeia that imitates the high-pitched sounds made by small animals — birds chirping, mice squeaking, or even the sound of rubber wheels turning. You can use it alone or reduplicated as 吱吱吱 for a continuous effect. Note: the dictionary pinyin lists only 'zhī', but the actual pronunciation of the full reduplicated form is often 'zhī zhī'.