Pull the cover and do a visual inspection on the carrier assembly. If you can pull the pin and inspect the spider gears, friction unit and the pin for excessive wear. You can do this without removing the carrier or changing any settings on the rear. I have seen some posi's make noise when they needed serviced. If you do not see anything wrong after inspection, service it with a good posi lube and a tube of powered graphite. Test drive it and see if the noise is still there. From the discription you gave, the carrier may need rebuilt.
RH