You appear to have eliminated almost everything with your testing - good job. I'm going to say axle(s). They appear to be the only thing spinning that would only appear at a specific speed (as opposed to Engine RPM). Coming from the car (and not your hands) would further indicate the rear of the car.
Otherwise I will go with Pucker Flutter
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Martin
2011 Cayman S (Gone) - Hardtop Blechster
2006 Cayman S (DD)
2016 Mazda CX-5 (Her DD)
2002 Boxster S (Gone) - Ragtop Blechster - Pura Patina!
Dorkiphus: I buy it for the articles