Could it be the winding up of the flywheel on the impulse starter, My memory is going can't remember the proper name, basically you spin up a lump of metal till it's going really fast then engage it with the engine and it gives enough impulse to turn it over a couple of times.
They'd all gone to clutched starters by the time I got on them so it's a bit fuzzy.
After you've been on them a week you couldn't hear any of that stuff anyhow as you were deaf.
