Won't be the upper pillows as rear struts are not Macphersons on the late model Subarus like the earlier GC and GDs.

Chev, best is to find a day next week and disconnect the rear links to see if the noise is from the links. Rear Shocks on the late model Subarus connect to the same arm as the linkages and this is usually the issue.

Will take a look at the rear parcel shelf too. Edit: Oops you have already Sussed that one out.

Cheers.