I recently purchased HM JPX 925s from a Mizuno dealer and it has the same serial on every club so it's odd you don't have a serial. Also, the whistle like sound you're hearing might be normal. I notice it too when I'm swinging my clubs, almost like a jet is flying by when you swing down fast enough. If you look inside the back of the club (the gaps), there's a "sound bar" (i think that's what they call it) which is what I am assuming is making the whistle sound.