I have heard this on a couple bikes, now I have it on my 426. When I lift the back tire and drop it, there is a loud clunk, especially if I swing the rear to the side a bit. Sounds like the shock and coil is loose and clunking almost. Probably a hearing of some sort, the rear wheel bearings are just about brand new so that's not it. Anyone have any ideas? Sound like something I should be really worried about?