[enrm6]

I've just had a similar problem, as Mark mentioned, this was a rear upper suspension mounting broken. In the estate the way I found out was that if you open the tailgate and then push down in the rear deck, one or the other corners of the deck panel nearest the seats will lift up. It is difficult to actually see unless you remove the panel. However this should only be a rattle when hitting bumps, unlikely to hear it on smooth roads. If it is this, it is a cheap fix - the part is only £10 or so.

Russ.