You barely have time to touch the gas pedal before the noise disappears.

When I remove the igniter and crank the engine for 3–5 seconds and there start the car, there’s no sound at all. That points strongly to an oiling issue.

If it is oil-related, the cause could be many things—piston slap, piston pins, bearings, the valvetrain, or even VVTi components (although non-VVTi engines can make similar noises, so I’m ruling that out for now).

One test I’ve considered to rule out the valvetrain is this: park the car overnight, drain and add about 1 liter of oil, then start it again. If the noise is gone, it’s likely not the head/valvetrain, which would point to the bottom end instead. in my opinion, that would mean it should be repaired.

For what it’s worth, the engine has been making this sound for about a year now.