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Kenwood Chef A703C making jerky noises. How to fix?

I have a Kenwood Chef A703C Mixer. (The 703C is an Australian made product, it is a A701 type with a semi-electronic circuit board, type A901 early version. Picture available here: https://kenwoodchefrestore.co.uk/Kenwood%20Chef%20Models1.htm) It uses the A901 repair kit for when the capacitor fails (https://kenwoodchefrestore.co.uk/shop/index.php?route=product/product&path=67_101&product_id=178). I have replaced installed a repair kit that I got from Ebay (https://www.ebay.com.au/itm/203526248918). The motor now makes a jerky noise. It is most noticeable at slow speeds, and is still audible at higher speeds. It is similar to this sound: https://www.youtube.com/watch?v=4Mi5XuS_ZMQ&list=PLk1LSVb9IBQw9I53vjr28VbjwUSATYC7g&index=13 I have removed the play as suggested by one person and tried cleaning the what I think are the contacts referred to bay another person. Neither worked. The brushes have been replaced. Anybody any idea what the noise is and how to fix it?