Haha I just had this problem if it's only 5th gear at speed your hearing likely 1 of 2 things wind noise over the mirrors try folding them in just to see
Front diff is whining
I had this problem an found out for me it was the wind and mirrors others have said they tried that but no fix and found out it was the front dif but was not a problem and not damaging anything
Google it and you can see tons of threads about it.
Hope this helps you