Vauxhall Vectra c 2003 sxi petrol - Metallic Engine Noise - Mike1201uk

Hi I'm wondering if anyone could help me, I have a vauxhall vectra c 2003 sxi 1.8 petrol, the car drives well and pulls well, It has a metallic noise coming from the engine bay it sounds like an exhaust rattle I have had the Cat changed and whole exhaust checked and many new parts added (exhaust rules out), this noise happens on start up and when under acceleration I have no idea what is causing it, no lights on dashboard. can anyone help please?