Well it's normal with mine anyway
Do you notice it going over bumps (even small ones) but only at low speed, not say above 40mph?
I'm beginning to think it's a combo a few things like the rear door and the back seats.
The rear hatch door is definatley too light and/or flexible and doesn't have enough sound deadening. If you sit in the back with the rear seats down and knock on the inside plastic with the side of your clenched hand you'll hear a booming sound like a drum.
Same thing if you hit the rear seats when they're up - boom boom !
So going over bumps puts a wee impulse through the hatch/ rear seats and that's when you hear it.
That's my theory anyway
You could ask your dealer to have a look to try and adjust the hatch lock and the rear seat catches but I don't think they'll ever fully cure it. They may be able to improve it though so if it bothers you have a word.
That's my theory anyway. Normally it doesn't bother my but every now and again it really drives me mad
Does everyone's MINI do this then?