Diagnosis
That's the brushless motor's fan assembly failing. Debris has gotten into the fan shroud or the fan blade is cracked.
How to Fix It
Remove the battery. Use compressed air to blow out the rear air vents. Do not spin the fan with the air. If noise persists, the fan assembly needs replacement. This requires disassembling the rear housing.
If That Didn't Work
The noise could be from the planetary gearset. If it grinds under load, gears are likely damaged. This is a full gearcase repair.
Heads Up
Running it with a damaged fan overheats the motor. You will cause permanent damage to the stator windings.