The rattle was coming from the fender extender, there is a piece of rubber that keeps it from rubbing on the bracket and it was missing, so I put a new piece in and it's all good.
I've got a rattle in the same area on a 2011 Street Glide. Was the rubber missing from the center mounting point on the bracket on the inner fender? I've chased this sound all over the back of the bike and can't locate it. I really thought it was the seat bolt in the center of the fender but nothing I've tried there has stopped it.