Michelin tire issue

[69x22]

I bought Porsche rated 335/30/20s for the rear of my 57. When im just easing down the road. I hear constant pinging underneath the car, the wheels end up with little rock pebbles in them. I pulled the tires off and noticed that the underneath has been sandblasted from just 80 miles of driving. These are 200 tread wear tires the same that are on my Camaro which doesn't have this issue. What do you guys think the issue is? I have read the reviews on Tire Rack and I don't see any issues from the Porsche guys. These were on sale at Tire rack.