I might have a possible explanation. If you open the hood and look closely at the cowl panel at the base of the windshield, there are several metal clips that hold the leading edge of the plastic, vented panel over the wiper drive mechanism.
If one or more of those clips are missing a vibration or rattle can result. These clips hold the two sections of the cowl together that are the protective shroud for the wiper transmission and also channel water away from the engine compartment.
This could be a source of the noise. When i created a ducted and vented hood for my 2011 Fiesta there were several conditions that occurred in making the hood panel direct airflow. One of them was rattle or fluttering sounds and the other was humming or whistling.
I eventually resolved the noises by removing the underhood sound deadening and caulking all the air gaps between the hood frame and the outer panel. Including tape over every hole in the hood frame except for the prop rod rest. Then the hood was quiet. I will guess that your noise originates in the cowl area.
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