Hey i've done 2 of these already it's common as muck on this model i done 1 last week cost £600 4 parts alone 2 genuine rear strut tops and 2 standard spec non genuine kyb shocks. I get on well with my local subaru dealer and they advised kyb's as standard spec replacements as thats wot they use (p.s there is a subaru technical bullitien about this problem stateing in it that it is a poor manufacture problem) hence the dealers not fitting genuine subaru rear shocks!.
P.s i had on the 1st car i done a rattle was still there after doing the rear shocks not as bad but still noticable and found after hours of stripping everything out the boot 2 of the retaining screws and nuts had came of the rear spoiler and were rattleing about in the tailgate (which is another common problem according to subaru)
good luck m8