Diagnosing rear diff whine

I have a turbine hum that sounds like a bad rear diff (turbine noise - not unpleasant but def brand new). Does it automatically indicate I need to replace it or is there some pre replacement things that could be diagnosed done? I havent dumped the oil yet to see if there is metal but either way that would not fix it. Wondering if there is something in between perfect and f**up that could still make the hum.

Truck has 140K, 2006 4wd, 6 cy.

Thanks!

 

I would check u joints on driveshaft as well as the carrier bearing. They can also make these kinds of noises.

 

When does it make the noise?
When you are on the gas, or when coasting?
At what speeds?

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