Common problem it will need splitting and cleaning easy to do remove the 6 screws at the bottom of the box 3 either side and it will come apart don't dismantle the gears. Plenty of tutorials on the internet
Take it to Steve at Bath Road Motorcycles, Bristol, and, for a fee, he will smooooooth it out. This is a common fault, I've had it on two boxes, and this man sorted mine. Tell him Shane sent you, be nice, drink BRM's fantastic coffee, peruse their immaculate machines and chat to Chris and Darren. Great people who care about their customers whom I highly recommend.
Common problem it will need splitting and cleaning easy to do remove the 6 screws at the bottom of the box 3 either side and it will come apart don't dismantle the gears. Plenty of tutorials on the internet