2002 Vauxhall Zafira rattling

On first starting the car from cold there is sometimes a horrible knocking noise from the engine which lasts only a few seconds. If you then sit and increase the revs there is a definite tappet rattle about 2500 revs in neutral. On driving the car the rattle is noticable when the engine is not under load i.e when declutching to change gear , this is all more noticable when the engine is not up to operating temperature. The rattle is still there when the engine is warm but is not as noticable .The engine has had regular oil and filter changes and the mileage is 60000, it uses a bit of oil but no more than what it did 40000 miles ago.The performance does not seem any different and the engine is not running rough. Any ideas ?