Just a wild shot but check your springs aswell. We had a corsa sxi a few years ago with a similar problem and it turned out the suspension spring had snapped a bit off and it was rattling.
Didn't affect the handling of the car, just rattled a lot.
Hi we bought a Corsa sxi and had an annoying ratling noise, couldn't locate where it was coming from, it went back and forth the garage and turned out to be the door latches, once fixed noise dissapeared. common problem apparently.