The noise might be amplified or exacerbated by other things, such as stiff u-joints in the driveshaft, or some kind of sympathetic vibration. It's a good idea to get down there and check everything you can - shock mounts, exhaust brackets, u-joints etc. Some 8 1/4 rear ends can be pretty noisy anyway, though. Mine has howled for years. It was a little worrisome when I got it at 110K miles. It's a little louder now at 248K, but a little less worrisome.
A change to synthetic lube can sometimes quiet it a little, but if it's not too bad I would just keep an ear cocked for change, but wouldn't worry too much.
I also notice that it's louder with the seat down, among other things, so if you've just put the seat down or just cleaned 10 years of crap out of the back, it may not be the rear end itself that's changed sound.