Noise in rear diff while turning is most likely the limited slip differential clutches alternately grabbing and releasing. This is usually a sign of contaminated or worn out gear lube. GM factory fill gear lubes are nothing but cheap stuff. Weather its new car or not makes no difference, the gear lube is the problem.
They have countless bulletins out over the years dealing with limited slip noise/chatter/banging etc in every model. I had to have the gear lube changed 2 times on my CTS with posi because it make noise when turning. I made them install AMSOIL Severe Gear 75W-90 back in May of 2009 and I have NEVER heard a noise again, nothing. Its always the gear lube but GM is stubborn and continues to use low-bidder lubes. Get rid of the GM brand gear lube and use a good synthetic like AMSOIL, Royal Purple, Red Line.
Btw – My brand new 1970 Chevelle SS454 did the very same thing, been a positration nuisance since they outlawed whale oil as a additive in gear lubes.
