Here's what I have so far: When the stitch width is 0-1 it does not make any noise when moving the handwheel back and forth by hand with the the thread take up at the top; with the stitch width 2-5 and the same movement there is a definite metal on metal tap/clank sound. I can eliminate the noise by pushing in where circled on the picture, so the noise, on my machine anyway, is coming from the needle bar mechanism. When I run the machine with various stitch widths I do not hear any metal on metal noise, but the whole machine vibrates a lot, especially at open throttle.
I learned there can be such a thing as too much oil, or at least too much in the wrong places, because I oiled the stitch width adjustment (circled in picture) and when running the stitch width selector will vibrate over to 5 if it's anything greater than 0.
And for Leon, there's my bench testing outlet put together just for this project.
I tried to post a video of the movement producing the noise, but the file type is not supported and I can't convert what I have.