2016 Ktm 125 engine noise

Hi all im looking for help, my son rides a 2016 ktm 125 it's done 38hrs totally standard apart from fmf full pipe hade new piston kit and little end bearing at 20hrs.. It has a terrible rattlle on the overun, example when closing throttle into corners or going over jumps, it is definatley from engine, as i have checked everything , put new o-rings in exhaust tightened everything, it has no loss of power and sounds great on full throttle its just when riding and off throttle any help would be appreciated as i self mechanic to save money but i have no idea where this rattle/clatter is coming from its not a knock it is as though there is something loose or rattling......