While the change in motor noise is most likely a motor fault, do examine the grinder itself. If the front ring is too tight, the mixer will grind (pun alert) to a halt.
If the grinder is adjusted properly, the motor’s worm-follower gear is the next thing to look at. It is designed to fail and save the motor from burning out.
The Service Manual can be found at:
++http://www.kitchenaidparts.eu/documents/pdf/service-manual/classic-svc-manual-new-email-version.pdf++