It's a lot more audible from under the car. Sounds like it's coming from just before the car. The speed of the rattle increases as I raise the rpms, but disappears after around 2000. I sounds like an engine knock but I know it isnt. It sounds like a loose hanger or bracket or something. I really have no idea to be honest.
Here's the video of the sound:

 

It your cats man. I have the same issue with my 08 coupe. I was worried it was a knock too. I put my car on a lift and its definitely coming from the cats, mainly on my passenger side. When its at idle it must rattle whatever is loose in there. I plan on unbolting my cats to see if anything falls out pretty soon.

 

Could also be the exhaust heat shield but my guess is the cats from only doing it at low rpm.

 

Did you listen to the video. I had another rattling sound that I took care of. It had a different sound.

 

Did you listen to the sound in the video? I had a different rattle coming from the heat shield that I fixed already. But I don't think this rattle is coming from the cat or a heat shield.

 

Yeah I did. It sounds really similar to the rattling I'm getting in my car. Best thing to do is get under it when its on to know for sure. I would not recommend crawling under it while on jack stands. I just checked it out when I got my alignment at firestone.