It's probably not the case, but I have to ask anyways, are your lug nuts tight? No offense intended, but sometimes it can be the stupid stuff. I've done it myself.

The rattle when you really get on it could be from the exhaust. Once, when I replaced my solid motor mounts with some rubber ones I got some weird noises upon really hard acceleration. It turned out to be a pipe hitting the trans crossmember. The clearance was really tight and the torque moved that pipe just enough.