Makes clatter noise when starting. 2009 cts
If from the engine, it could be valves before their lifters fill up with oil. If from the exhaust, it could be a heat shield or loose parts inside one of the components. Has the problem come on gradually? How long after the engine starts is the clatter is done?
Cadillac had a known issue with the timing chain back then. I’d suggest having a mechanic listen to it.