Not only a word in its own right, it’s imitative and “reduplicative” – which means that “rat-tat-tat-tash” is considered the same word, just lengthened out for more sound effects.

• 01.047 but a hard rat-tat on the hobbit’s beautiful green door.

“rat-tat, n., int., and adj.” OED Online. Oxford University Press, March 2015. Web. 1 June 2015.