GM has updated clutches that fix the problem for good  no matter how many times you change the diff fluid and add the additives untill you have the upgraded clutches installed the problem will come back  I hope for your sake you are still in warranty when it does !

[QUOTE=AlMuniz]There is a service bulletin number 070420002 that address this problem.  The dealers are quite aware of the problem.  To correct it the rear differential fluid needs to be drained and adding specified Limited-slip axle additive.  This is a toss up as to the dealer charging for the work or doing it under warranty.  The normal cost is $250.00.  Either way your car will sound a lot better if you get it done.  No more clunking from the rear after the car sit for a while and no more rubbing and clunking sounds on turns.  I had it done after I found the service bulletin and I would willing pay the $250.00. [/QUOTE]

When I had my 2008 C6 in for service (replacing air conditioning condenser because of a small crack) the mechanic was changing fluid in a 2007 Vette.  He told me about the service bulletin and I told him I heard a slight noise from time to time.  He had me read the service bulletin which said in some 2007 and early 2008's the clutch additive had not been installed properly or some such wording!  He drained the differential fluid and one that had the friction modifier included-no charge.