Very interesting ... I encountered a LOUD intermitent harmonic hum, actually more of a high-pitched raspberry, when driving across Kansas last summer. The wind was blowing 40+ from the side making it difficult to stay on I-70. I could never find the source of the sound. Since it only occurred in Kansas (not Colo. or Mo) I chalked up as bad Kansas Kharma. The windshield was replaced last winter, so the trip west will be test this summer.
Speaking of new windshields, what is with Subaru glass? The old one broke where the rear-view mirror mounts. Apparently, while removing her jacket my wife wacked the mirror and the inside ply of safety glass disintegrated and down came the mirror. I've never heard of that happening before.