The "clatter" your hear is a combination of the sound of the valve train, which is a fully mechanical system, with no hydraulic lifters to "dampen" it, and the sound of the fuel detonation as it echos off of the cooling jackets around each cylinder, but it is mostly attributable to the "harder" valves used in diesel engines.

You'll usually find that it's more noticable after you have either had a valve adjustment done, or done one yourself.

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Scott Longston
Northern California Wine Country...
"Turbos whistle, grapes wine..."