Adverb (fukushi), Adverb taking the 'to' particle, Suru verb
with a rustling sound; (rummaging) around
Onomatopoeic or mimetic word
隣の部屋では、夜遅くまで起きてごそごそしている音がする。The people in the next room stay up until all hours doing God knows what.
Also written ゴソゴソ