[jmk]

This can be totally normal for a Hurth 630 (ZF63) family gear. It is typical and not something to fret over. It comes from the fact that rotation speeds are not constant but fluctuate due to cylinder firings 4 times per revolution. This causes rattle that disappears at higher RPM. If you have one bad spark plug then the rattle can be worse, so check all your plugs. But keep in mind that it seems to show up on the Stbd side for Hurth gears because they are reversing gears and the port is actually not running identically to the stbd, but through a different gear train. Lots discussed on this elsewhere. A tight cutlass bearing can cause the shaft to deccelerate more between ignition events, leading to more rattle as well.