Based on my experience with a previous vehicle - i would point to one of the wheel bearing. I had a similar situation which took months to figure it out. At the end was one of the wheel bearings - with a strange behavior - no play, no grinding noise- only a resonating sound at above 70-80 mph - impossible to find it on the stationary vehicle. I found the issue only on test drives with 3 mechanics in the car - trying to find from which part of the car the sound was coming. Try this and let us know the results
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