[Xakaz]

It worth to think which attacks may hinder your foe's ability to scream without grappling his face or neck. Stabbing his neck (not blood vessels) may damage his vocal cords. And stabbing his lungs may impede his breathing, at least muffling his screams.
Probably, best way to remove sentry without helmet - is just Telegraphic AoA (Strong) to his skull with something heavy. At least in action movies, in this case your foe will quite reliably fall unconscious without making any shouts.