Clunk noise below the car

[under1rooftop]

Hey guys, I'm drive a 2004 Yaris RS (also known as a Vitz where i'm from) and i've been experiencing some issues whenever i shift it from 2nd to 3rd, or sometimes even on 1st - 2nd. This happens especially when i shift into gear too early and i didn't notice it till i actually turn my stereo off. A friend of mine told me it's nothing major but I'm just wondering what do you guys think of this since everyone here drives a Yaris.