It's a doe "blowing". Does and bucks do it usually when in a state of alarm. Sometimes they know what they are blowing at, sometimes they don't. It happens often when they are startled, see you, hear you or smell you.
It's often not a good thing for you, but when you hear it, the show is not over. I've seen and killed several deer after, just recently, having another deer blow in my vicinity.
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