Technical Clunk from clutch

[Mully59]

I have a 2013 fiat Ducato 2.3, when I push clutch pedal down then let it up I get a clunk what seems to be underneath the engine, I have crawled underneath and noise seems to be coming from inside a small housing close to gearbox, has anybody had this or can shed some light on it, thanks Alan

 

[rayc]

My guess is that it is the concentric clutch slave cylinder