1999 Dodge Ram 1500 rear differential noise when accelerating (Chrysler 9.25 differential)?

My rear differential makes a noise when I accelerate. It only does it if I have maintained a speed between 30-50 mph and then hit the gas to accelerate, and it only does it in 3rd and 4th gears. The fluid level is full on my differential and I have maintained it pretty meticulously. It isn't a grinding sound, and the truck doesn't lose any power, it is almost just a groaning sound coming from the rear diff. Any suggestions on what it might be? I took it to a mechanic and he told me it was a common occurrence and to turn my radio up, but I just don't think differentials make noise just for the heck of it. Just in case you need to know, it is a Sport model, 5.2 V8, 4x4, with a 5 speed manual transmission. Thanks for any suggestions.

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