I just took my newly-purchased 2016 Miata GT on my second 125-mile trip and because traffic is so light, I am routinely driving at 75-80 mph to keep pace with traffic between San Antonio and Austin. At this speed, the car produces a whining sound. I believe it's coming from the rear passenger side. The sound disappears at speeds below and above this range. The highway is asphalt. Any suggestions?

 

Try swapping tyres/wheels from another vehicle to rule out tyres.

Run it up on axle stands and use a stethoscope to listen to each hub.