Just to close out this 'thread', which I started nearly two years ago, I did finally diagnose the buzzing noise I could hear in strong headwinds or driving briskly (same thing). It was indeed caused by the windshield wipers that I had installed in CA years ago. They are longer than standard, because it was felt that the stubby little wipers that come with the car, looked ridiculous (and only cleared the lower part of the windshield). So maybe we now know why the car has those silly wipers - to test, I simply placed a couple of Kleenex tissues under each blade, so they were not touching the glass. A swift test drive confirmed that the noise was completely gone.
I guess someone at Fisker Automotive in the dim distant past had the same issue and our stubby little wipers were the solution. I'm now wondering if the rubber has hardened over the years and whether a replacement of the blades with nice supple rubber would allow me to keep these better-looking wipers (without the buzzing noise).
I am happy that this is currently the only issue I have with the car - a blessing.