Rear Differential Howl

If i pull the front air dam from my truck, ill get a slight howl at 40-45 mph. I assume its wind noise thats happening with it removed. I had reinstalled it, and its no longer there.

 

Two years since my last comment. 83k miles. What howl?

 

When I was a Toyota tech in the last century, this was a common problem with passenger vehicles.
There was a lube shop up the street that was changing out leaking driveshaft oil seals at the pinion shaft.

The rack monkey was using an impact wrench to seat the pinion shaft nut, and over crushing the crush spacer that sets pinion depth.
This ruined the correct pinion to ring gear spacing and trashed the differential.

At that time I was rebuilding so many differentials that we stocked rebuilt spares for rapid swaps.
I'd swap out the differential unit, get the customer out the door, then rebuild their core for the next swap.