I remembered this thread the other night, had an origin in for a light refurb. It screeched like hell when turned on, removing the belt had it sounding smooth. One of the brushroll bearings had seized solid.
Cover caps off, scraped all the hard black crap out with a pin, got it turning again. Re-packed with grease, back together, new belt and she was smooth and quiet again!
Just incase it helps!