Hi. I would name it 'reluctance' noise, 'magnetostriction' noise, periodic 'growling' noise if I have to guess words. I would not call it a buzz / hum.

Runs fine, noise disappears totally at the moment of switching it off and continues turning smoothly and silent a long time until momentum dies to a stop. It is not lack of lubrication. Not wobbly. It is clean. Noise is only when energized.

Any clues how to diagnose / fix / check ?

 

What's your guess?

 

Shading ring loose?
Loose Lamination noise?
Maybe the stator requires a dip in shellac?

 

What's your guess?

Needs some superglue? seeping into lamination gaps and coil turns...

 

I have a ceiling fan that would produce a very obvious hum noise once per revolution. It was quite irritating. So I experimented, and found that the glass shades aroud the bulbs were serving as vibrating megaphones. The first fix was a rubber band around the portion of the glass shade that fit into the shade support. One rubber band on each of the shades, and adjusting the tension of the retainer screws. That worked for about 25 years, although the bands did not last and had ti be replaced,

 

Dismantled the ceiling fan: applied cyanoacrilate glue to windings and laminations. Worked. Happy. Is back working quietly.