There is a piece between the two bears in my wheel that is getting losse and making noise. I'm not sure what it is because the manual doesn't say what it is, what is it and what does it do?

 

Are you referring to item #7 on this page Collar ? If so, that collar is what keeps the bearings from being squeezed together as you tighten up the Axle bolt. The inner race of the bearings press against this collar as a spacer. It maintains the gap between the bearings from side to side.