Expansion Tank hissing

I am having trouble with the cooling system on my 110 after driving even a short distance you can hear a hissing sound coming from the expansion tank. This started after it went for its mot, a new radiator has been put in as this needed doing anyway I also changed the thermostat. This has not stopped the hissing and boiling noises coming from the expansion tank. I am pretty sure that there are no air locks. Has anyone got any suggestions as to what could be wrong?

The header tank is faulty. It must have a small hole or crack along the joint. Replace it!

First off check your expansion tank cap but I think it might be time to get a sniff test and compression test carried out as it sounds like headgasket.

check presence of oil in coolant, take a whiff of expansion tank when engine is running, check expansion tank cap(ie the valve system should be working).

my bet would be a busted head gasket.
Adi