I had the same issue on my ATS-V. A bit of rattling on clutch release when I bought it at 13k miles that came and went. Never there when I wanted to show it to the dealer. Then at around 30k, all of a sudden started shuddering where I could not take off smoothly. It became unpleasant to drive. It would do that for a few weeks and go back to normal for another few, then return. I thought it was either clutch disc contamination or the throw out bearing binding. Ultimately that was the reason I sold the car. I didn't have anyone I could trust to pull the tranny out to deal with the clutch and didn't want to spend several grand on this. I never abused the car or burn the clutch. Sure a few doughnuts here and there, but most of the driving was normal take the kids to practice stuff. I suspect the previous owner (dealership owner's adult son) had some fun learning to drive stick.
