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Cold oil not circulating, yeah it probably left some crud that got inside a tappet. If not changed for many miles the oil filter might even have opened its bypass valve so not filtering. Don't drive without air filter.. but fine for investigation in a none dusty or gritty place. If you are going to leave it for any length time cover the inlet. I have used clean cloth, plastic bag and rubber glove over the years.

It sounds like a possible rattle from a hydraulic tappet. If the engine hasn’t been run for 4 years the tappets, or at least some of them, will likely of lost oil. If your oil light goes out on starting the engine and you’ve done a change with the recommended oil, it’s sometimes an idea to rev the engine for a few minutes, then idle again to see if it quietens them down. You can repeat this process a few times. Just letting it idle may not quieten them down. I’ve just bought a new set of INA tappets for my MK2 polo and the recommendation is run the engine for 4 minutes at 2 to 3K revs,then 30 seconds rest and seeing if they quieten down, if not repeating the process for a further 4 minutes. If that doesn’t work do it for 6 then 7 then 8 then 9 minutes, with a 30 second rests in between, in total 5 attempts to stop the rattle. If that doesn’t sort it then you may have a faulty hydraulic tappet/camshaft or an oil pressure issue. As I say this is the recommended procedure for NEW tappets, but it might do the trick, especially as your engine has been stood for so long. I emphasise you must ensure your oil warning light goes out before trying this.

Thanks. Yes sounds very like hydraulic rocker(s). You could take the engine cover off and use a "listening stick" to try to narrow down which cylinder(s) have the problem. Like a poor man's stethoscope, hold a long screwdriver against the cylinder head working left to right. Might be able to clean insides of the lifters with engine flushing additive. Or might need a garage to change worn / dirty lifters. Was oil real hot when you changed it?

I only hear exhaust for that. Can you record from engine bay? Probably the hydraulic tappets slow at pressurising, so the cam lobes slap the valve ends, rather than stroking them.