I have a 1958 Craftsman lathe that I converted to variable speed with a DC motor. Wanting to know it's speed range, I decided to go old school with a disc tachometer. I used discs calibrated for 60hz and they work fine with shop fluorescents. See the video for more details. Included below is a disc pattern I made and is free to use. The youtube video page has a link to the full size graphic.
The knocking sound was a loose set screw.