gunrunnerjohn posted:My guess, and it's only a guess, is the tach sensor is missing a stripe now and then. This would cause the processor to see a slightly varying speed and might trigger a sound step. Note that all of these sound files have steps, I don't know for sure how many sound steps the PS/2 prime mover sound file has.
Replacing the tach tape might help? Worth a try. Think I have one laying around.