Ever since my first oil change using full synthetic DEXOL grade oil, I have had an intermittent rattle at startup when the engine is cold. I did not experience this on my 2011 Regal 2.4L before the oil change. It seems that it is more likely to happen if the car is started with either the air conditioner or the defog on. Any ideas as to what is causing the rattle? It usually lasts 2 - 3 seconds and then disappears.