The next thing I looked at was the tacho drive, this is a worm style gear and runs at 90 degrees against the corresponding gear on the end of the idler shaft.
The tacho gear seemed to have an excessive amount of vertical play, suggesting the noise could be this gear vibrating up and down? The gear has an upper and lower washer, which the manual highlights to remember to reinstall and an oil seal, which was super hard.
As a test I rebuilt the side case and refilled the fluids without the gear, I cut a rubber finger of my workshop gloves to cover the tacho gear drive hole just in case something went in the motor or oil came out.
I have compiled some clips with various stages to focus on what was the rattle or what is just standard engine noise. I also recorded my MB8 for an engine noise comparison but I didn't include that in the vid, so just MBX.