New rattle

My Octy has gained a rather strange rattle in the last couple of days. It occurs at idle and is very annoying/concerning.

Sounds sort of like the noise you get if you drop a cheap biro onto a desk and can be heard from outside and inside the car, when in the driver's seat its almost as if it's coming from just infront of the engine bulkhead on the driver's side of the car.

It doesnt seem to be directly related to anything mechanical, as in it doesnt change its rythm exactly with the engine.

At idle it rattles regularly but once on the move, at constant reve the rattle is irratic.

Definatly more of a mechanical under-the-bonnet sort of noise than a trim rattle (hence my slight concern)

Ideas?