Sprint Gearbox Noise

Hi guys. Hope you all well. New to this site although iv had two sprints over three years. This one is a 12 plate minter although it's developed a grinding noise from the gearbox by the front sprocket. Iv had the chain off and run it on the center stand. Same noise. Iv held a long shaft screwdriver against the box and listened through it and can hear the grinding inside. Just wondered if there was anyone who'd encountered this and perhaps would be kind enough to pass on their wisdom/experience regards what it is and how to fix it. Thanks
Mark

 

Thanks for your reply. I agree with you. Think I'm looking at an expensive fix. Boo!
Thanks again.
Mark