2009 Cadillac CTS clattering noise

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.