Considering the weight of the engine over the rear tires of all 911's, along with the very slight toe-in and positive rear camber of the rear wheels/tires when the alignment is set to spec, ALL rear tires on 911's will wear somewhat unevenly as they are used. Wear will be more pronounced on the inboard portion of the tires and some cupping will develop due to the slight toe-in of the rear tires. This is what will eventually cause tires on 911's to begin to rumble/howl/growl. I've seen it on N spec and non-N spec tires. Due to the aforementioned alignment settings of 911's (including 911, 964, 993, 996, 997 models), the rear tires of these cars will never get quieter as they wear. Fwiw, the alignment settings on my 996 are spot on set to Porsche specifications.
