I always thought that the rolling of rrrrr's in the Miqo'te and Mithran npc dialog was an indication of purr-rattled speech. Kinda like how the sound of a cat's meow rattles with vibration when a cat meows during a purr.

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Also, while I am sure its coincidental, the voice I chose in the voice options for male Miqot'e has a slight throaty purr-rattle in the "YEAH" sound of the "cheer" emote.