Rattling tends to be more noticeable with metal components, but my guess is perhaps some body clips may be loose around that area. I'd do a quick underbody inspection and verify the clips are all tight and present. If you've taken your Crosstrek off-road, there is a good change some clips have been damaged or missing, and maybe some minor damage to the front bumper may be present.
"Why would I want to profess my feelings for some 3D girl?" - Tomoya Aki from the anime Saekano