I had a rattle in the cargo area that I managed to solve. It happened during rough roads and bumps. I'm not sure if you have the same problem as me, but mine was a metal strip above the window in the cargo area on the driver side. I had a heck of a time trying to pull panels to be able to access behind the headliner so I could get a wrench in to tighten the two bolts.I'm experiencing this same problem on my outback that I purchased today. Any help would be appreciated.
If this sounds like your problem, be careful not to rip your headliner and wear gloves when removing trim, your hands will get dirty from the grease they use on the weather strip and you don't want to stain your headliner.
You can't miss it, it's a metal strip attached to a wire with two bolts. At least, that was my problem.