I just bought this 2011 SS conv. with only 19000 mi on it in Aug. and about a month ago I started to hear a grinding noise coming from the rear end when I turn corners or pull into the driveway like the old posi's use to do when they needed an additive. So I went and had the oil changed in the diff. and the proper additive put in but I still hear the grinding noise when making turns but not as bad. Any Ideas?