my favorite grunt phrase would have to be "toss and munch", ive caught em saying that a few times as they were throwing a plasma grenade
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I either like "I'll bite your kneecaps off" or "good thing that food nipple's waitin' for me at the starship 'cause man, I've worked up a big grunty thirst" you could hear about this on halo.bungie.org in the tips and tricks section and in the easter egg section, its called the food nipple grunt