A cold Diesel
or a low RPM
Diesel which also has very low exhaust
temps so it can be considered to be a cold motor
will rattle because it’s a compression
, meaning the whole charge goes off nearly instantly as it all hits the ignition point almost instantly.
A spark plug
engine doesn’t normally rattle because the spark plug
initiates the burn, that propagates from one point and spreads throughout the combustion chamber.
If a spark ignition engine’s charge is initiated by compression
, it too almost instantly combusts the whole charge, that’s called detonation and they will also rattle, and because they are not designed to tolerate it, the piston head
often fails from it.
Diesels often quieten down at higher RPM
as there is enough heat in the combustion chamber to initiate the fuel
burn as soon as the fuel
becomes present and it is a smoother flame propagation similar to a spark ignition engine.
Common rail motors can introduce a small injection event prior to the main event and make enough heat be available so that the fuel begins to burn as soon as it’s injected, sort of emulating a fully warmed up Diesel at higher RPM.
Old style injection systems at much lower pressure can’t do that, so they rattle.
It’s a normal operating condition of a mechanical injection Diesel.