2005 optra has intermittent top end rattling noise, seems more often when its cold but it goes away after a min or so, any ideas?

Sounds like your lifters are bleeding out when the engine is shut down. It goes away because they pump themselves back up when engine runs. They can rattle while running as well. I would get some engine oil treatment and run that in your oil. A good brand is Seafoam run that in your oil and that should clean out your lifters.