My guess would be your air cleaner clanking once against the valve cover.

Hold the air cleaner and have an assistant start the car. I bet that makes it go away.

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'85 300D, 'Lance',250k, ... winter beater (100k on franken-Frybrid 3 Valve Kit)
'82 300D, 'Tex', 228k body / 170k engine ... summer car
'83 300TD Cali Wagon 210k, wife's car