There was a window in late 90s/early 2000s in Europe where certain countries company car tax rules made it often more economical to run the diesel model of a lot of German cars and as a result Diesel sales took off for a while. This is why you saw the innovations like Common Rail Injection etc and large increases in refinement that became prevalent in a lot of diesel cars of this era. There was so much improvements in diesel tech in this decade because of this. There are some legendary diesel engines like BMWs 3.0 straight six M57 etc from this time - the great low down torque made this one of the best engines in BMW's range, period.

With the more recent European emission rules, and the massive "dieselgate" scandal of course, there is simply far less appetite for further diesel innovation nowadays and in the future. Pump availability at the gas station of Diesel itself is also now pretty poor in North America vs Europe too. VW/Audi also had a great reputation for refined Common Rail diesel engines in this period as well, ironic given dieselgate.

The common rail stuff got so refined Audi even offered convertible A4s with diesel for the first time, diesel cabrios were largely unthinkable before due to lack of engine refinement. Generally these engines were still noticeably diesel sounding at cold idle, but once up to temp after 5 minutes it got a lot harder to tell unless in the cabin looking at the low rev limit on the dials.

Is that what gives diesels their trademark rattley sound, the injectors?

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.