On my way home last night, I was just thinking to myself and started to become very curious as to why.

I remembered dismantling a diesel engine because the big ends went. And remember putting it all back together again. I saw the engine design very much similar to petrol engine bar the head in some respects. I started to think but couldnt find an answer. Maybe I've missed something fundamental or its more complicated but the question I want answered:

Why do diesels make the noise that they do? The chuggging noise? The classic dug-uh-dug-uh-duh-uh-duh-uh? What is it that makes them make this noise?

Thanks