Whenever I have the bike a gear combination that has the lower jockey wheel either below or in front (towards the front of the bike) of the upper one, I hear this rattling type sound. It sounds as if it is coming from that lower jockey wheel, but I can't be 100%. I can move that wheel (in fact, I can move them both) from side to side slightly, meaning that they wobble in my hands given some force. So, is this wobbling likely to be causing this rattling type sound? How do I stop them from being wobbly (I'm sure its not good to have wobbly anything)?