I have it on my 06 hemi. It is now getting louder and its from the differential. The mechanic claims it is not abnormal. I don't think it is. I have logged my first complaint and will do so again. Especially if it continues to get louder. I have never been in a vehicle with 12k on it that made this noise.

 

All 90 degree gearsets (ring and pinion) will display a whine of some sort between about 1700 to 2500 driveshaft r.p.m.
This is most prevalent during float (maintaining speed, not accelerating or decelerating) or deceleration.
At that time the backsides of the ring and pinion are in contact.
If the noise disappears upon throttle reapply (light throttle) it is generally considered normal.
Keep in mind, I am refering to a whine that usually requires smooth road/radio off, or at least volume low, to be audible.
Anything much louder should be monitered.

It is interesting to see this question asked.
When most cars became front wheel drive, the question didn't arise since they do not have a 90 degree power path change.
I wouldn't be surprised if those who notice this noise didn't come out of or currently own a front driver.
Something to think about,
Rob