I've been looking for 6 months to find somebody with a 2011-13 wrb sti to swap with no luck.
I went to the local dealership and asked the parts department if anybody there has installed an sti spoiler on a wrx. Believe it or not, someone did a month ago and they just recently took it off because they said it rattled like crazy.
So they were trying to talk me out of it and said don't because it will rattle even when you turn your music up. To me that sounds like they did a crappy job installing it and THAT is why it's rattling.