It's the hubs. It's common on our cars and I'll bet there are few cars WITHOUT it (but yours looks worse than most, I'll say that). There's a lot of threads on this.
Here's a DIY to get rid of it
http://www.e90post.com/forums/showth...light=rust+hub
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Current- '21 430 , '19 911, 07 M6 ,'17 Jeep Wrangler, "23 Bronco OBXluxsquatch