Rattling Noise from Zetec Engine????

Hi all, hope someone can help me!! I have just purchased a Ford Mondeo Zetec 2 Litre Petrol 02 plate. It has a black top on the engine (aparently there are 2 types)? Anyway the car is in immaculate condition for its age and I bought it from a genuine guy who has just received a company car so got rid of this one. For the last week I have noticed a rattling/tinkling noise coming from around the cam belt/manifold area, hard to pin point. I cant hear it while moving, just while stopped on idle and it gets more rapid if I rev it a bit. Oil and tensioner have been checked by 2 mechanics but they cant see where its coming from and advised me to take it to ford who apparently charge about £90 an hour!!!!!
The car is running fine and pulls fine, no smoke from engine, no other sounds, Any ideas before I remortgage my house to get ford to look at it......PPLLEEASEEEE!!!
Thanks in advance :)