DIAGNOSIS: A foreign object—like a pebble, LEGO piece, or chunk of debris—is lodged in the main brush chamber or the air channel leading to the dustbin.
FIX: Empty the dustbin completely. Then, remove the main brush and inspect the chamber. Shine a light in and use tweezers to pull out any hard object. Also check the side brush for obstruction.
IF THAT DIDN'T WORK: The noise might be from a failing wheel motor or a damaged main brush bearing. Manually spin the wheels and the main brush. If one feels gritty or doesn't spin freely, that's the culprit and will need replacement.
HEADS UP: Run the vacuum daily on a quiet setting to hear early signs of trouble. A persistent grinding noise will quickly damage internal gears. Stop using it until you find the source.