From reading the forum messages, there's all sorts of things that go wrong with 3rd gen rears. Fortunately, most problems are relatively cheap to fix. The parking brake shoes came un-bonded from the backing plates in mine somewhere around 45,000 miles. The noise from this wasn't very loud, and they might have sounded like you describe. It was a $45 fix, but you need to put it on a lift (or a really good set of jack stands) to change the shoes. And it was a real pain in the butt.
As far as other possibilities - For wheel bearing problems, the noise can be made to come and go depending on if you're going straight or turning. Differential noise - I think - usually will get worse if the temperature of the differential is hot (they heat up if driven at highway speeds for some distance) and won't change whether you're turning or going straight.
