DIAGNOSIS Debris has entered the main motor fan or the motor bearings are beginning to fail. The sound is air being forced through a blockage or dry metal.
FIX This requires disassembly. Remove the battery. Use a Torx T10 screwdriver to remove the screws on the motor housing cover. Carefully lift the cover. Inspect the small fan intake at the top of the motor for hair or string. Remove any debris with tweezers. Reassemble.
IF THAT DIDN'T WORK The motor bearings are worn. The entire motor unit (part VCA-M75A) needs replacement. This is a professional repair.
HEADS UP Never use the vacuum without all filters installed. A missing bin filter lets debris get sucked straight into the motor, causing this noise and failure.