This is the 6422nd most frequent Russian word.
шум
Noise.
Here, 'шум' refers to a general and undefined noise or sound in the environment.
На улице был сильный шум.
There was a loud noise on the street.
In this case, 'шум' describes the sound specifically associated with leaves moving or rustling.
Они задели деревья, и возник шум листьев.
They brushed against the trees, producing the rustling of leaves.
Here, 'шум' conveys the auditory perception of the flowing water in a river.