1.8 zetec engine help with knocking sound

Hi firstly great forums and as i am now a proud mondeo owner i will visit frequently! so hi to everyone! :content:

Anyways i picked up a mondeo 2002 1.8 zetec the other day and all seemed fine drives smooth and nice,

BUT i was parking it up tonight and noticed a knocking noise coming from it, it sounded like something hiting the inside of the engine casing, it was not a tap rather a knock.
it was NOT going to the revs of the engine more randomly at idle and sounded abit like a diesel.
I opened the bonnet but it was very hard to pinpoint where it was coming from.

I revved it slighly and it was still there but not as loud so i turned the engine off and went indoors.

i went out about a hour later to start it up again and no noise revved it abit still no noise run it abut 5 mins still no noise lol oil is good and so is water, it starts and idles well not perfect but very well.

I have heard horror stories of Big ends but would theynot make the noise all the time?

any ideas?

thanks in advance