Hi
I had the same problem and took it to several garages and none of them knew what it was and it was doing my headin, so, one day Idecided to take the back wheel off and see what it could be, and you know what it was?
The rear pads were lose when the handbrake is not applied and the pads are hitting on the rear brake discs, when you go over a ramp or on an uneven road surface you hear the rattling sound.
Get the rear pads sorted, what I did is, I took the pads out and placed a small think peice of metal behind the pad and I don't hear the rattle no more.
Cheers
Faz