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It sounds to me is a problem of brushes.
If I am right, they are using Ringfield type motors and the noise as you
described does like ones I fixed before.
One or both brushes constantly hit(s) the slots in the motor commutator.
My solution was simply to swap the two brushes.
I have met three similar problems and sorted out all three by swapping
the pair of brushes. It does no harm at all to try it in a ringfield type motor.
But if it is not a ringfield type motor, you may have to file down the meat of
the brushes a bit to sort it.

You also need to do a good clean of the slots in the motor commutator carefully with a sharp toothpick
(they'll be full of carbon from the brushes if the noise was truly caused by brushes).

Clean the commutators copper face as well as you can with a cotton bud soaked in
thinners or alcohol. Be careful if you use thinners as plastics don't like them.

If you have some some super fine wet and dry it can be used to polish the face of
the commutator until it shines like a mirror. Use 1200grit or better glued to an ice
lolly stich for this, and be gentle and "even" in pressure. Polishing the armature
face evenly will improve the efficiency of the motor a lot.

Before you swap the brushes or after file them, it is also worth giving them a good clean - soak in meths for a
while and then give them a good clean with detergent - let dry totally (overnight in
a warm place).

Please kindly let me know if this solve the problem. I will be very glad to hear a good news.
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