2001 Espace 2.2 DCi losing coolant

Hi everyone. I hope one of you experts can help me.

I have a 2001 Espace 2.2 Dci manual and it's losing coolant.
There's no visible leak
No oil in water
No water in oil
No white smoke from exhaust

When engine is hot and switched off I can hear a gurgling sound coming from the front of the engine. It seems to be coming from below the airfilter box.....

Anyone any ideas before I get the spanners out?

2001 Espace 2.2 DCi losing coolant