Sorry, I can't really tell. Only thing I can do is advise getting under there and shaking things around, it sounds like something is loose. To me it sounds like something with the exhaust, like maybe hitting the underbody and that's why it only happens over bumps or in first gear (first gear is when your exhaust pipes shake the most.) It sounds like you have aftermarket exhaust, maybe it wasn't installed exactly right.
Hopefully other people have some different ideas.