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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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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