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?

This can help us, help you.

 

Ok turns out I had a bad carrier bearing. Phew. Not too bad - got installed 2 years ago and failed - put a genuine toyota one on. I brought a stethoscope which made diagnosis trivial.

 

Curious how you used that to test the carrier bearing? Rear wheels off the ground and hand turn the drive shaft?

 

Wheels off the ground, truck against a tree and blocked, put it in drive. The mechanic stethoscope came with a long rod - I held it against transfer case, carrier bearing bracket and rear diff and the carrier bearing one sounded like grinding - the other were smooth.