Listen to the Thirsty Grunt talk about his eating habits
Halo Combat Evolved THIRSTY GRUNT – Achievement Guide
Complete Skull and Terminal CHECKLIST(Halo 1-4 / Google Docs)
During your escape on the Warthog in The Maw, you will come across a bridge where Cortana will tell you to stop. Continue driving to the next left turn. After that you will see an open door on the right. Go inside and take the next right. You will find the Thirsty Grunt there.