I had this same problem. It started at around 27K miles on my car with the stock tires. It would happen even when I only had 1 person in the back seat as light as 135lbs. I had noticed that my SVT had settled a bit and was a bit lower that they rest of the NorCal group out here that were stock. I took it to the dealer and they reported that the hight was in spec and they could not get it to make the scratching noise that you described. I found the that the rear passenger tire sidewall was rubbing on the edge of the fender under corning. I took it back and test drove it with them and made sure that I had three people in the car, and it rubbed. They said I had too much weight in the trunk (I have about 150lbs of stereo back there just two amps and 1 12" sub.) It got much worse when I got my 215x45x17 tires. I believed that the springs had settled a bit too much and the dampers were shot, so I replaced them with Koni struts and even lowed the car with H&R's. Even now that the H&R's have settled in and are much lower than stock, I never rub any more, even with two 200+ guys in the back.
