I'm going to do my best to explain what is happening with my 2008 G6 GT with the 3.5v6. Sometimes when shifting from park to drive it will shift very hard into gear. Once in drive, it will continue to shift hard. When this happens the engine idle is different than normal and is much louder than normal. The car doesn't throw any codes for the CEL to come on. Funny thing is when I hook a code scanner up, it reads no codes like I said. But even with no codes present, if I select clear codes anyway, the engine idle changes and car starts shifting fine. I just want to reiterate that no codes show and the CEL light does not come on. This hard shifting will also reset itself if the car is left off for a period of time. Does anyone have any idea what this could be?