Phone up Logitech, explain to them the wheel is making a grinding noise when turning, and they will send you a complete replacement unit. They will ask you to return the plugs off the old unit to render it unusable, but you can keep the pedals intact as a spare set. When I damaged my old one last year this is what happened.
If you wish to void your warranty..to me it sounds like the geartrain is damaged, perhaps when turning fast once it skipped a few teeth and damaged them, maybe the motor moved slightly, throwing the gear mesh off, etc. If this is the case theres really nothing you can do. If you do take the cover off, check for any obstructions in the gears as this is also a possibility.