Differential noises on 1990 Porsche 928

The differential starts to act up after some normal driving. I get grinding noises especially when turning at low speeds. Have tried changing the fluid including using additives etc... it seems like there is damage in there. Has anyone experienced this and know what it might be? Is it worth rebuilding and is there someone local that specializes.

Asked by for the 1990 Porsche 928
Did you find out what the problem was, did you have to replace the differential?