I have a 2010 Forester, the heat shield is a POS. That is most likely your rattle.
Solution is to get about 6 to 10 hose clamps of various sizes (Amazon, stainless Steel) and crawl under there and hose clamp that rattling heat shield and the Catalytic converter shield too. It's a pain, I know.
My constant use in the salted roads of West Virginia and Virginia and the Pot holes of Washington, DC made my Forester extra noisy.
Solution is to get about 6 to 10 hose clamps of various sizes (Amazon, stainless Steel) and crawl under there and hose clamp that rattling heat shield and the Catalytic converter shield too. It's a pain, I know.
My constant use in the salted roads of West Virginia and Virginia and the Pot holes of Washington, DC made my Forester extra noisy.