Engine sound on cold start on a 2014 Vauxhall/Opel Corsa. Ticking sound usually goes away after 2-3 min after being cold

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That doesn't sound at all like piston slap to me, I'd characterise what you have as top-end noise. Piston slap is twice per crank rotation and is a lot more 'hollow' sounding.

These are never the most smooth-sounding engines out there, and that one seems like no exception. Assuming it has had regular oil changes, I don't think there's anything specific to worry about there. All I hear are some aged hydraulic tappets loudly doing their jobs.