For me, it was a grinding/whining noise that was especially noticeable if I took my foot off the gas between 40 and 50 miles per hour. But even at other speeds, and under acceleration, an unnatural noise could be heard.
It sounds so bad I expect it to catastrophically fail at any moment, but just the spider gears look bad, not the ring and pinion axle gears. I think the bearings are producing much, if not most, of the noise as well.
I am having the rear end rebuilt next week and installing a TrueTrac. I hope the T makes it.