Wiktionary
rat-tat-tat
interj. 1 The sound made by knocking on a door with a hard object (such as a door knocker). 2 The sound made by firing an automatic machine gun.
Usage examples of "rat-tat-tat".
Calico forgot it all during those ten delightful minutes when, with his heart beating time to the rat-tat-tat of the snare drum, he swung prancingly around the yellow arena.
He was just fiddling hopelessly with the cufflinks on his third shirt when the rat-tat-tat came at the door.
I listened and decided which wall it was coming from, and then I picked up an old wooden potato masher that was lying around and went rat-tat-tat on the wall myself After that there was no more knocking.