2004 Cadillac CTS Vibration in engine

There is some vibration that is considered normal, here is some literature to describe rough idle common to the V series.
Rough Idle
The 400 horsepower LS6 V8 engine derives its awesome power through a combination of displacement, stroke and tuning. A key element of this combination is giving the engine better "breathing" ability through unique camshaft to valve lift and duration. The efficient process of getting the air/fuel mixture in and exhaust gases out of the combustion chamber is achieved through this higher valve lift and longer valve open times. The result of camshaft design results in what is commonly referred to as an unstable idle or the random roughness of the engine. This is normal and does not indicate a concern.

In addition, this vibration or roughness can be enhanced by worn or collapsed engine mounts. If you also have some clunking during acceleration, moderate to hard, then this would be more of a possibility.