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.
When I walked through the fallen leaves, they rustled.
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.