adverb, adverb taking the 'to' particle
rustlingly; with a light rustling sound
Describes a soft, dry rustling sound, like leaves, paper, or fabric rubbing together. Often used with と.
落ち葉が風でかさこそと音を立てた。
The fallen leaves rustled in the wind.
彼女は紙袋をかさこそさせながらお菓子を取り出した。
She rustled the paper bag as she took out the snacks.
Onomatopoeic formation from the light, dry rustling sound it represents. The exact origin is uncertain.