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:

 

Interesting but try the easy stuff 1st. Check the exhaust and manifolds for holes. Do you have a rough idle or cel?

 

Nope idles fine, no waring lights either.

 

That sound's like the cat's or exhaust heat shields...

 

Well I took off the heat shield on the y pipe before the resonator so it isn't that.