Keep at it and best of luck with the "search the problem out" mission. I love my 403A and don't remember hearing anyone with a discouraging word to say about any of them. When she came to me she had been in my parents' home and was completely gunked up with glop from Mother's little, black cigar sort of cigarettes and Dad's pipe. Why the two of them did not die of lung cancer passes my understanding. Anyway, I can also authoritatively say that my worries about the gunk invading the machine's finish were unnecessary. And the paint even came clean!
I hope that you've discovered the problem was a simple one and are now "un-probleming" your way into no more grinding. Thank you for the thread. I want to keep mine and use her until one of us goes. Problem is who should get her then. I love mechanicals.
Pat