My WAG....speedo cable. Never had one whine on me, but I know a few riding buddies who've had this problem, and the usual sympton was a noise that changed pitch when speeding up or slowing down. Try lubing it. Either that, or maybe you're getting an aerodynamic noise coming from somewhere in your new setup. Try removing the windvest and see if it goes away? Or how about the cable routing? Did you run the wiring through the new bars and maybe the wind is blowing over the holes like you do when you blow on an empty bottle?
