Adrian,
The car isn't moving otherwise a good guess. Its an engine noise and yes it souns like clock.
Could it be one of the external valves opening and closing like egr of which there are two valves.
How is it if you increase engine speed slightly.
What about the belt tensioner?
Is it there when the engine is cold and/or hot ?
Engine mounts, any sympathetic movement of them in time with the ticking?