The red, being firmer, will transmit some more rear diff noise into the car. It would be awfully hard to quantify how big a difference it really makes. And the rear diff is mounted to the rear cradle, which has 4 big rubber mounts to mount it to the chassis, so it's not like the diff noise would be directly transmitted into the car.
With that said, if you're at all concerned about NVH, then I would go with the black, just to play it safe.
With that said, if you're at all concerned about NVH, then I would go with the black, just to play it safe.
