Bad rear differentials 
2017 Toyota Tacoma Double Cab SR5 4dr Double Cab 4WD 6.1 ft. SB (3.5L 6cyl 6A)
Bad Rear Differentials
 My rear differential made a loud howling noise on the first day of ownership and showed signs of failure after 300 miles, so it was replaced.  The new rear diff makes the same noise.  Toyota states the noise from the rear diff "is normal for all tacomas."  I then drove other Tacomas at the dealership.  Guess what, they all make this noise that you can hear over the radio at 50-60 mph. If your looking to buy i would stay away from these trucks.  Google tacoma noisy rear diffs