Diagnosis
A foreign object (like a rock, piece of Lego, or large staple) is caught in the main roller brush chamber, or the roller bearings are failing.
How to Fix It
Immediately stop the robot and flip it over. Remove the main roller brush by pressing the two release clips on the ends. Inspect the roller and the chamber it sits in for any hard debris. Clear it out.
If That Didn't Work
Manually spin the roller with your hand. If it grinds or feels gritty without any debris present, the roller unit itself is damaged. Order a replacement main roller assembly.
Heads Up
Running it with this noise can damage the motor gearbox. Always do a quick visual sweep for large debris before sending it into a new room.