Purchased this vehicle new with 417 miles. From the first, there was a slight tick/rattle upon cold start. Issue became increasingly problematic until it sometimes required multiple attempts to start, loud knock was heard, and entire vehicle would shimmy for a few seconds upon start up. We had it in to the dealership multiple times with varying diagnoses, including: needs oil change and new filter; GM says these vehicles just have engine noise, and we have no way of knowing whether or not this is excessive noise; we can't reproduce the problem; and nothing's wrong - that's normal. Finally, at 18,071 miles, it was taken in again and discovered that it was the timing chain. The chain was slack and the gears had wear. The dealership replaced everything under warranty and the problem was significantly better, but not solved. It then continued to worsen again through several trips to the dealership, where previous diagnoses were repeated. Finally (again), at 43,384 miles, we took it in again and it was discovered that the timing had jumped or the chain had stretched. We declined to have the dealership fix it and took it to our own mechanic to repair. We're on our 3rd timing chain at 43,000 miles. Mechanic believes it's a defect in the chain design or composition, but unable to find any recalls or reports on it.