A Rattle, but any suggestions where from?

[Forbez]

Guys I hope you can help me, there is a rattle on my missus' 52 plate (03) Mini Cooper S.

Its a metalic type rattle sound, and I have narrowed it down to happening when the rear wheels hit a bump or dip, and doesn't occur when the fronts hit the same bump.
The sound seems to come from the rear drivers side of the car.

I've looked under the car and can nothing that seems loose, even the heatshield on the exhaust seems fine.

I've also checked the manifold heat shield in the enigner bay.

While narrowing this down, i've noticed that the rattle doesn't occur if I brake slightly when going over a bump. I have also tried this with driving with the handbrake on its first click, and this seems to stop the rattle.

I have searched and searched but can find nothing like it.

Can anyone suggest anything that this may be? Hope you can help, this is driving me nuts now, and the missus is fed up with the car.