Just to summarize:
The issue is most noticeable above 60 mph.
Your father doesn't notice it
The issue persists even after re-greasing wheel bearings and changing fluids in the differenetials
The sound does not increase or decrease with throttle
Things that come to my mind:
Based off what you've posted and my brain - it is something that builds in pressure/resonance as you increase in speed but isn't affected by acceleration.
- Fuel pump is right behind you under the 2nd row passenger seats
- Catalytic converter whine or a loose connection on your exhaust
- Idler pulley
- EGR valve noise
Usually drive train noises would usually change on and off throttle because the throttle applies pressure to the system. That pressure changes the sounds usually.
Was there ever a definite resolution to this? I’m having the exact same issue and my mechanic can’t seem to figure it out, but he seems to think it could be a hardened transmission mount or something. Front wheel bearings have been changed, all u-joints in the drive line have been changed (they were going bad), diff fluid is new, topped off, not leaking etc.That really does sum it up. Those are some great points thank you so much. I know the catalytic converters are a little loose so that could be my problem. I will look at that today. Thank you so much!