Please help my zt water pressure

Hi all I have brought a mg zt 2004 1.8 turbo with 126k on the clock. Did all the normal checks when I had it a weeks ago BUT.... Having a look under bonnet straight after a 15 mile journey and heard hissing from the cap on expansion tank and water pipes feel stiff :( re checked oil and water earlier and no mixing going on.



Could this be Hgf? :( I have paperwork for uprated head gasket, head skimmed etc which was done 2 years ago but not sure if it's done a lot of miles since then. Maybe 25 to 30k



Any ideas?



Oh and not lost oil or water. Water is same level between high and low and no mixing in oil or water

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