DIAGNOSIS Debris (hair, string, carpet fibers) is wound tightly around one or both of the brush axles under the robot, causing friction.
FIX Flip the robot over. Remove the front wheel and the two brush modules. Use the included cleaning tool (or scissors) to cut and remove all hair and fibers wrapped around the brush ends and the axles inside the robot. Reinstall everything.
IF THAT DIDN'T WORK The noise could be from the front wheel bearing. Remove the front wheel, clean any debris from its socket, and spin it by hand. If it's gritty or doesn't spin freely, the wheel module may need replacing.
HEADS UP You must clean these brushes every 1-2 weeks on carpet to prevent motor damage. It's not a design flaw, it's maintenance.