Anytime I go on a long drive my check engine soon light will come on and it runs fine until I put the car in park and turn it off. It almost sounds like it's sitting in 3rd gear and just slamming down thru gears when I cut the power to the motor. It's an extremely violent banging sound that will make anyone within a 10 foot radius snap their neck to make sure the trans didn't fall out. I'm at a loss and I can't ever get it to happen at the dealership....

 

Even if you can't re-create the actual condition, the dealer should still be able to read the check-engine-light code that you say you're getting.

 

Sounds like the car is running extremely rich but definitely get the code check out. Autozone does it for free, just ask for an OBD-II scanner.

 

Do a ride-along with the technician once you can reliably re-create the condition.

Get this fixed immediately!

 

The banging around could very well be a broken motor mount--that doesn't act for the light but the noise.