I'm having a similar issue but I have aftermarket coilovers. I checked and recheck the suspension but still can't figure it out. I have the Cobb exhaust hangers so I really don't think it's my exhaust hitting. I'm thinking about putting my gopro under there to see if I can find anything. It's only the rear right which is installed exactly the same as rear left and I've checked the shock again and again with wheel removed.