Try all above and...
Check the rear seat is firmly latched. Bit of black tape round the catches can help here.
Check the parcel shelf is clipped in properly and make sure the black plastic bits the parcel shelf clips into are screwed down tight.
Check the steel panel inside the boot isn't missing any screws or clips. If you can hear the noise when you strike this panel, take it off and make sure washer tubes and wiring isnt rattling about and hitting it. Have a look for any areas the paint has rubbed off where the steel panel attaches. Make sure the foam tape round it is intact.
Grab the exhaust tailpipe and give it a shake, can you shake it to make the noise? Possible wrong/broken rear rubber mountings but more likely the bracket infront of the 2nd cat has broken and put more stress on the rear mounts.
Reach under infront of the rear wheels and grab/shake the rear anti roll bar links. Any wear will show up as movement in the ball joints. Only works if the car is on the ground or MOT type ramp where the weight is on the rear axle. You can check the anti roll bar D bushes by grabbing the bar either side near the bush and grabbing/shaking it too.