Why does my guitar make a rattle/buzzing sound?

My guitar (acoustic epiphone) makes this weird sound whenever i play it, my cousin has the exact same one and i can play perfectly fine on hers but for some reason mine makes that noise. It did it ever since I first got my guitar but i just ignored it and then bought new strings and it still makes that noise. It only happens when i play a chord for some reason. Does anybody know why?! I really need to get this fixed!!