Recently I have noticed that my engine has been making a ticking noise. The best way to describe how it sounds like is like an old diesel engine type of rhythm.

When the car is cold and idling I can hear an occasional 'tick' coming form the underneath of the car. When you are at WOT the rhythm begins to start straight away, every time. It also becomes very loud when the car is about to stall (under 500 rpm)

I have knocked around the engine bay looking for loose things but everything seems to be solid although this ticking sound is coming from the left side of the engine and I cant seem to find out where it is coming from. (The same side where the window washer fluid bottle is)

There are no performance issues at all. The engine is running perfectly.

Any ideas would be great.

Does the rhythm changes when you rev the car?
If so, that is a rotating part of the engine.
Try to see if there is any brake in the auxiliary belts.
My Peugeot has this kind of ticking from it's belt wich is a bit torn and one of the threads is showing from the belt hitting the case of the engine..

To be honest, the rhythm doesn't seem to change with rpm, but it tends to get louder the more you open the throttle. (Even if the engine rpm doesn't change)

For example: 5th gear and wide open throttle driving slowly you can hear it mostly.