Grinding sounds and slower mixer speeds

Question edited for clarity.

The drive train probably need cleaning and re-greasing, but there is also a gear that wears out.

https://www.google.com/search?q=Kitchen+Aid+Mixer.+How+to+fix+a+grinding+sound+when+switch+is+on+Medium+speed

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It sounds like there may be an issue with the motor in your Kenwood Chef Premier mixer. This could be caused by a variety of factors, such as a malfunctioning motor, a problem with the drive belt or gears, or an issue with the electrical components of the mixer. To troubleshoot the problem, you may want to try cleaning and lubricating the drive belt and gears, as well as checking the electrical connections and fuses.

Did you remove the carbon brushes. If so they go back in any of four ways. But only one way works correctly. If you did not remove the brushes then you need to set the speed timing on the speed control switch at the rear of the machine and you can find a YouTube video on this exact exercise on YouTube of course. You may have also reattached the wires to the speed control switch to the wrong connectors hence no work.

you probrobly have a gear inside missing some teeth, so the slower you go the more you can hear the grinding, i would un plug it and take the housing off and check the gearbox, maybe the intermediate gear is going bad.

could be the result of a bad bearing,either that or something has caused the offbalance of the rotor.it should be repairable,if you take it to some repair workshop,unless you would want to take it apart yourself.

It sounds like the Worm Gear is going out. The faster the motor runs,
the more damages the Center Gear is grinding away the teeth of the
Worm Gear. Things start to slow down, and eventually they will come
to a halt. It's time to replace the Worm Gear. Also make sure to use
new fresh Kitchenaid Food Grade Grease with the new replacement.

Sounds like a classic case of the gears jumping out of sync. This is an issue that needs to be handled by a repair place or sent back to KitchenAide. The motor on a KitchenAide is very strong and it sounds like when the cap for the accessory fell in it must have somehow gotten jammed, although I really don't see how that would be possible unless there was something else in the mix as well.

Replace the motor with a similar size and voltage. It should say on the motor what size it is.. Take the gear off and save it for new motor.

No change available. The motor speed if fixed and the 3 speed transmission has fixed gear ratio's. Look at the new HL300 model. It has a stirr speed which is slower than first speed.
Does your mixer havea safety cage? If not, you will need to install one before too long , or the machine will not comply with current safety standards. Put the cost of the safety kit towards the HL300.

The repair manual says that the mixer can make this grinding sound at slow speed, like stir or 2. This is normal, especially until it warms up. This is the sound of the gears turning at slow speed.