You probably don't have p-trac there and p-trac will make noise like that on turns first. The spider gears pretty much don't spin going dead straight unless you have oddball tire sizes on both sides to differ greatly.
If u-joint then it will be fairly high frequency to match the u-joint spinning which is faster than rear axles turn. If slower then look for something as simple as a rock stuck good in the tread, that can do it.