My 2015 Cadillac ATS makes a noise in towards the rear of the car when I start ot ? It only has 10k miles on it. It s clunking moise

It may still be under factory warranty which in this case is a good reason to take it to a Cadillac certified dealer. It sounds like something is moving that isn't supposed to be. If it's happening when accelerating, it may be something with the rear axle mounts, differential, suspension, etc.

It happens only when I start it.

Does it have the air ride suspension in the rear? If so, maybe something there causing movement?

Yes sir it does.

It may be something popping/knocking as it's bringing the pressure up. Does it do it every time you start? Like, if you run it for a few minutes, turn it off, and start back up does it do it then while the air pressure is still nominal? Something may be rolling/snapping in the suspension while it pressurizes. Have someone else start it up and watch the rear axle and see if it shudders at all during the pressurizing process. Either way, if it's still under warranty, take it in while you can. If I remember correctly, Caddies come with 3 year / 36k mile warranties on everything.