adverb, adverb taking the 'to' particle
Describes a dry, light rustling sound, like leaves, paper, or small objects being moved. Often used with と or as an adverb.
落ち葉の中を歩くと、がさごそと音がした。
When I walked through the fallen leaves, they rustled.
引き出しの中で何かがさごそ動いている。
Something is rustling around inside the drawer.
Onomatopoeic formation from the reduplicated sound がさ (light rustle) and ごそ (similar rustling), mimicking dry, light rustling noises.