Not an expert or a mechanic but in general find engines to be "more clicky" when low/lower on oil - had generally thought this to be the sound of the valves actuating. With less oil the sound is more pronounced. Maybe the "lower pitch" and the rest of the sound has to do with the high compression ratio (and exhaust or lack thereof). TDI engines can sound super clicky compared to larger truck engines

Pretty good discussion here as well (cool didn't even know there _was_ mechanics stackexchange): https://mechanics.stackexchange.com/questions/26513/whats-th...

Edit: would assume 4-cyl pistons need to actuate/fire ~twice as much as an 8-cyl engine at equivalent rpms? maybe that would explain smaller engines being more clicky idk

No.

While there is some noise from the injectors (in some gas-direct injection engines you can hear them as well, if you listen carefully) the sound you are hearing in an old diesel, that handful-of-marbles-in-blender-clatter, is due to the more abrupt combustion in a diesel engine in general (only air is compressed, and the fuel burns almost instantly on injection) and imprecise/poor timing of the old mechanical injection systems. Modern common-rail electronic-injected diesels are almost as quiet as gas engines.