So after 4 weeks of relatively problem free riding, other than a fuel tap leak, my first major mechanical bike problem. Seems my gearbox is on the way out. it whines in 1st 2nd and very slightly in third now. It clunks aswell even when switched off and just rolling. Sounds as though the gears aren't meshing. I have no problem changing gear whilst riding and it still pulls like normal. However when I downshift coming to a junction or bend, not aggressively at all, it clunks and knocks, sometimes so much so I can feel it as well as hear it. So from that info I'm pretty sure its on its way out, please so correct me if I'm wrong lol would love a simple fix haha but I doubt that. So my main question is how much of a problem is changing a gearbox on a bike? Anything that needs to be done specially? Changed gearboxes on cars and rebuilt car engines but not sure if there are any significant differences with a bike. Any info would be appreciated.