Without actually hearing the rattle this is probably a shot in the dark, but I am absolutely frustrated with this. I've had a couple buddy's listen to it and we can't isolate it.

It doesn't rattle under normal acceleration. Only when accelerating hard, and it rattles for each gear.
I have also lifted the rear end off the ground and rev'd the crap out of it with no unusual noises.

It is definitely coming from under the car, and our best guess is somewhere in the rear half of the car.

Ideas?

 

my rear exhaust hanger behind axle on passenger side was rubbing together and causing rattle then loud high pitched noise. I would check there. I had to take the part that mounts to body and notch back some of the metal with my grinder, now no more noise. Same issue, when jacked car up couldn't get to make noise, only when on ground and actually driving, happened to find just out of dumb luck. Good luck.