I've had a pine needle slapping the back glass before and I swore it was something in the front. Sometimes these noise aren't coming from exactly where they sound like they are, it could be something under the hood, or in the grill.
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Mirrors are a common source, out of position cowl trim, I’ve even seen grills where they fit to the hood. Wind noises suck!I seem to remember an old thread someone using this procedure. If I recall it ended up being the plastic valance panel in front of the windshield or the DS mirror.