What you're describing is probably piston slap. If the engine has been overheated at some time in its life, it's not uncommon for the pistons to swell, and collapse the skirts a little. After that, especially during warmup, there will be enough piston skirt clearance for the piston to flop around and make a knocking noise. If that's what's happening, ignore the noise. It is annoying, but not dangerous.
Jerry

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