Some form of metal filings/shavings/a metallic sheen/etc. is pretty typical of most aluminum wheels. Personally I hate Shimano pads, they seem too hard and unforgiving, and seem to accumulate grit in them too easily, whereas every time I would brake, it would basically sound like metal on metal grinding. I am a much bigger fan of Kool-Stop pads, they are much quieter and softer, but not so soft that they require overly-frequent changing. As an O/T aside, for carbon rim brakes, I've found Reynolds cryo-blue to be the best (of those I've tried).