What Irish said ^^ and I guess what BD posted.
Or another remote possibility is a rust hole on the end of the cowl near the hood hinges. If the
the 86 brings in air from under the fender that would be a place to look. If it brings in the air from the heater ducting then I don't have a clue.
One question though, are you absolutely certain it is coming from the air vent and not from behind the kick panel or above them? Just curious.