OKay, so I've been wondering this for the longest time, and I've spent the past week searching the forums with no success, but what causes that ticking noise in ultegra/dura ace/campy cassettes? And why do they only have it on higher end components, but not Sora/Tiagra/105? To make it even more confusing, why do cheap mountain bike cassettes have it too? I mean, don't get me wrong, I love the noise. In fact, it's one of my favorite things to listen to on rides (I know, weird).

Someone please enlighten my ignorant self!