Isn't it just the cutest sound in their language? I just love it when they do that, like my Lily when she's greeting me by saying that.
Or, when you approach them while they're resting (which I know you shouldn't do, but sometimes I just can't help it ), they'd sound like "mmmrrr?", like a question? I wonder what that means in our language! (most likely something like "Can't you just leave me alone, mom?")