Onomatopoeia!

It looks like a Japanese word, but it's not. "Onomatopoeia" is an English word that refers to all those goofy expressions like "choko-choko" and "poro-poro" that you hear in everyday Japanese speech. The list of onomatopoetic expressions is almost endless, but learning a few will make your Japanese sound more natural. (With thanks to the folks at MIT, from whom I shamefully stole this list.)