While I haven't had this problem myself, I've read that some others had this very problem with their brand new Koralias. Believe it or not, simply disassembling them and reassembling them was all it took to quiet them back down for those people. I don't know the exact reasoning behind this, but I would tend to think that it's just because the impellar or the bushing on the impellar shaft wasn't seated fully during the initial assembly. It's worth a try -- there's not much that comes apart on them, so you'll only have a few pieces to reassemble. Just keep an eye out for the thin bushing that's on the backside of the impellar magnet -- it's easy to lose.