咕噜 (gū lū) is an onomatopoeia that mimics a rumbling or gurgling sound — most often used for stomach growling when hungry, but also for water bubbling or a low rolling noise like thunder or a cart. It typically appears reduplicated as 咕噜咕噜 for continuous sound, or compounded with 叫 (gū lū jiào) for 'stomach rumbling'. The syllables are neutral in tone in natural speech, but pinyin maintains the original tones: gū lū.