I have a silly question. You're someone with a tremendous amount of knowledge about electricity. So taking apart a simple motor like this doesn't pose a challenge. Why not take it apart and see what's making the noise?
Is the shaft rattling because it's missing a washer or two on the shaft? Are the brush holders rattling because they were installed incorrectly? Are the brushes in need of replacement? Are the brushes rattling inside the holders because someone installed brushes that were too small? Are the ends of the shaft in need of a drop of oil or some Vaseline to lubricate the motor? I suspect that motor hasn't seen any lubrication for at least 50 years. Has the copper on the commutator worn down leaving the mica proud and are the brushes bouncing on the mica?
I had the last thing on a Singer motor and what was needed was 10-15 minutes with a fine saw (an Exacto razor saw equivalent bought on the web for maybe $8.00 including three blades) to undercut the mica. After that job was finished the motor went from making a racket to purring like a contented cat. If you go the route of undercutting, don't bevel the edges of the copper, leave them sharp and undercut the minimum needed to ensure the brushes don't bounce.