So its definately in the motor if it does it in park...nothing in the engine bay that moves or can jump around while not moving.
I'd lay $5 your check engine like is probably a knock sensor getting tripped from the rattling the engine is making.
Do you know the broom handle stethoscope trick? I use the kind you can inscrew from the broom head and stick the end straight in your ear.
You can pinpoint engine noises that way. I wouldn't drive that mother ****er if I were you untill you figure out where the noise is coming from, trashed motor wouldn't be fun. I'd start by the base of the intake where it meets the heads. The next thing I'm pointing at is a lifter, push rod or rocker. Those will make a hell of a racket at low rpm if any of them came unglued. Start hunting around w/ the broom handle, it wont cost you anything. Just make sure you dont jam the end in the serpentine belt while its running. I've seen that happen before. Good luck!