Odd clanking sound

[Fatchimp]

Hi,
I have a late 2006 Megane 1.5 DCi 106 with 83k miles on the clock. Recently it's developed a strange clanking sound which I can't pin down to anything specific. I've even had it into a mechanics who could hear it but just couldn't place the source.

The noise is most prominent with the engine at tickover and makes no difference whether my foot is on the clutch or not. The noise is the same whether stationary or if I dip the clutch whilst moving. When on the accelerator I don't hear anything at all which leads me to assume it's something in the drivetrain. However, if I pop the bonnet with the engine running I'm hard pressed to hear the noise anywhere around the clutch/gearbox. There seems to be some up/down play in the clutch release fork (I assume that's what it is). Does anybody know how much play there should be here or any other ideas of what this could be?

Thanks for any help that can be offered.