Does it go away in a second or two, or in a few minutes?
If seconds: big end bearings are shot. Time for a new engine.
If minutes: it's piston slap. Worry not; it's normal and engine will run--
quite literally--forever with it.
Actually, there is another noise that's also normal: valve clatter. It's
higher-pitched than big-end knock, and clearly comes from the upper end of
the engine. This will go away just about the same time as the oil light
goes out, or about a second to a second-and-a-half.
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