Adverb taking the 'to' particle
with a slamming noise (e.g. door, telephone receiver); with a banging noise; with a clash; with a clank; with a bang
Onomatopoeic or mimetic word
ガチャンと言う音で私達はびっくりした。The cracking sound startled us.
Also written がちゃん、ガチン、がちん