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diesel engine compress no compression in petrol engine
SHARP POWER PULSES OCCURING DURING COMBUSTION IN DIESEL ENGINES
It's because diesel fuel ignites under compression, rather than by a spark, so burns rather than ignites, is less controllable which results in diesel 'knock' which accounts for most of the noise you can hear.
The actual combustion noise itself, is not very dissimilar to that of a petrol, a lot of the vibration from a diesel engine comes from the high pressure fuel pump and injection system.