Mission: The Maw
Near the end of the game, there is an easter egg known as the "Thirsty Grunt" who is a special dialogue grunt that appears on any difficulty.
You'll find him during the warthog section of the mission after the bridge where you see foe hammer. From this bridge, continue into the tunnel where you will go downhill and take a slight left turn. During this turn take a look on the right wall and you will see an opening. Go inside of it. Once you do, follow the narrow tunnel, and again, look on the right hand side where you will see another opening. Get out of the warthog and go inside. At the end, you'll find this grunt, and also get the Thirsty Grunt achievement.