murmur, rustling, or bustleDescribes a soft, continuous sound made by leaves, crowds, students, or other groups of people. The の connects the source of the sound to ざわめき, meaning a murmur or rustling sound.
JA: 森の中で、風と葉のざわめきに耳を澄ませた。
EN: In the forest, I listened closely to the rustling of the leaves in the wind.