Welcome
I would not worry about the mileage as many like myself do not use the Z as a daily driver. I am probably going to be just over that 9k when I hit my 4 year mark. (too many other competing vehicles & the canyons are in my back yard). As mentioned before it could be the tires grabbing the groves. Does this happen on other roads? Are there any mods to the car? Lowered, wheel spacers ?