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