Engine Clicking And Tapping Noises problems of the 2008 GMC Canyon

Purchased vehicle new with 417 miles. From the beginning, there was a ticking noise at start-up, which gradually worsened to a loud knocking and vehicle shimmy for several seconds, sometimes requiring multiple attempts to start. Originally, it was only on cold start-up. Multiple visits to the dealer resulted in incorrect diagnoses or no diagnosis. Finally, at 18,071 miles, the problem was diagnosed as loose timing chain and worn gears. All were replaced by the dealer under warranty. Problem was significantly reduced but not solved. It then began to again worsen through multiple trips to the dealership, until 43,384 miles, when it was once more diagnosed as the timing having jumped or the chains having stretched. We declined to have the dealer replace everything with us paying for it, and had our own mechanic do the work. Our mechanic believes there is some sort of timing chain defect in design or composition, which keeps causing/allowing the chains to stretch.