I haven't had a chance to listen to this from under the car, but the sound seems to be coming from the clutch area. The sound happens most when coasting to a stop in gear, but as you can see in this video it's happening as I rev the car as well, and sometimes during actual acceleration.

I've tried some research and so far the most likely thing I've found is that some gear teeth broke off and are making a racket in the transmission. I really hope this isn't the case, so if anyone else has any other ideas please let me know. The car is making about 180hp to the wheels with the turbo.

The car also has low compression in cylinder 3, about 130psi or so. I'm swapping the engine this week. Maybe the rattling could have something to do with that? Not so sure.

https://drive.google.com/file/d/0B5Oa90mXzaTPSldQVTNxNldhUkU/view?usp=sharing"]https://drive.google.com/file/d/0B5Oa90mXzaTPSldQVTNxNldhUkU/view?usp=sharing