2002 Nissan X-Trail noises from engine

Dear sir I am an owner of a NISSAN x-trail 2002 2.0 litter petrol, and when I start up the engine it makes a rattle noise for 3 to 4 sec and desappears, after that when climp to an easy hill or when the car is loaded this noise appears again(from 2000 to 2500 rpm) and there is a small power reduction. After that if I slow down a lot, the engine works better after 5-10 minutes, seems to me like there is an issue somewhere in the timing belt area of the engine and something like the lubricant get to hot and the variable timing parts do not work properly. Oil changed and also my mechanic does not have any idea, could yo please point me something, from you experiense?
thank you Spiros.