I had the same noise and check engine light, started up the engine and shut it off about 3 times and the noise sounded like 20 pigmy's with sledge hammers in the top end. I started the engine back up and same noise this was when the car was under warranty so I reved the engine and the noise stopped. I took it to the dealer and they could not find anything wrong and surmised that carbon depsoit had come loose in the valve train. The car then had only 4000 miles and now 6 years later it only has 14,000 but with an occasional short trip at 120-130mph I have not had this problem again.