I’ve noticed that big, diesel things like locomotives and busses are really loud and shaky when idling but get noticeably quieter when they start to move. Why?
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Lower frequencies propagate further. Inside the motor it surely gets louder when it speeds up, but the higher frequencies are better absorbed/blocked by the material around it
It’s also the design. Car manufacturers are constantly improving designs for not only functioning performance but also smoothness and quiet. On large engines, that is less of a consideration. Compare a modern diesel pickup to a piece of industrial equipment. The design of pilot injection and other improvements have made them practically as quiet as a gas engine.
Cause you don’t notice it. Inside the can of a big truck, the noise doesn’t go away at higher rpm, and gets much louder than idle. It does sound smoother as it gets faster until it doesn’t. The road and wind noise so help cover up some sounds at speed, but the engine is now moving a bunch more air and it’s not getting any quieter.
Large diesels, especially older ones, have fixed injection timing. That means that the diesel is squirted into the cylinder at the same position on every stroke.
When the diesel is turning quickly, the time it takes for the fuel to burn is similar to the remaining time to the top of the piston stroke. When the diesel is turning slowly, the fuel fully combusts before the piston reaches the top, and the high compression makes a tinking noise. It’s the same noise that cars make when they’re ‘dieseling’, igniting early due to overheating.
TL;DR: The noise at idle is a side effect of tuning the engine for most efficient performance under load.
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