First, don't panic. Lots of earlier Mirages, my 2014 among them, make what some say is considerable racket on a cold start. I think your is likely flashed and fixed, you can look it up, I forget what link to use. Mine is flashed, still noisy. Horrible compared to my 2017.
Now, some 2014-2015 Mirages had legitimate piston fit issues. They would slap on a cold start and the racket died down but never completely went away. Those engines need replacement. Check with your dealer only after you're listened to many, many youtube videos of noisy Mirages cold-starting.