DIAGNOSIS That grinding is almost always a foreign object—like a small piece of gravel, glass, or hard debris—caught between the vibrating mopping plate and the robot's chassis.
FIX 1) Remove the water tank and mop pad assembly. 2) Look at the silver mopping plate that vibrates. You'll need to detach it. Push the two plastic locking levers on the sides inward and lift the plate off. 3) Now inspect the cavity where the plate sits and the underside of the plate itself. Remove any debris. 4) Also, check the mop pad's velcro area on the plate for stuck-on bits. 5> Reattach the plate, ensuring it clicks securely into place on both sides before putting the cloth back on.
IF THAT DIDN'T WORK The noise could be from the plate's motor or gears failing. Without debris, try running a mopping cycle with the plate detached (just set the robot on the floor). If it's still making a bad mechanical noise from inside the robot, the mopping module itself likely needs service.
HEADS UP Always do a quick vacuum run before mopping, especially on hard floors. The mop plate sits very low and will suck up any small, hard objects that the main brush might have missed, causing this exact problem.