Ford Mondeo knocking noise/rattle when under load.

My Mondeo has developed a metallic knock noise when driving under load and sounds a little like "pinking" when accelerating, it sounds more when the engine is at operating temperature. I thought that it may be the Aux Idler wheel so replaced the wheel and belt with ford Parts. I also changed the HT leads and coil pack with new pattern parts(good Quality)but the noise remains. Car has done 78k miles and has been serviced by me every year at 6-8000 miles since i bought new and i have always used Ford items at service time although i did use the more expensive brand of ford oil at the last service as the local dealers said that that was all they now stocked. Is there any history of camshaft wear or associated parts on these engines or perhaps the oil breaking down prematurely, or have i overlooked something more obvious.
Thanks for any helpful replies.