Gurgling header tank

[itsalwaysme]

Hi
Ive a vehicle that is going fine- not over heating or using any coolant but every time I switch it off I can hear the coolant gurgling around.
Don't think its head gasket problem.
Theres no cap on rad, only on header tank and ive replaced that.
Where the hose goes into the top of the rad theres a bung/bleed screw- if I slacken it until the coolant flows out and then run engine up to temp after its cooled I can take bleed screw out and no sign of coolant until I slacken header cap and then coolant appears.
Should this be the case? and whats the cause?

Sorry for such a long post and thanks.

[HustleRussell]

i've had similar and it turned out to be a small and difficult to spot coolant leak. Or it's boiling because of a bad water pump, thermostat or expansion tank cap. Or it's pressurising with combustion gasses.

[itsalwaysme]

Thanks Hustlerussell.
Coolant doesn't seem to be boiling, im leaning towards a coolant leak or something but cannot find anything.

[HustleRussell]

Mine was from the thermostat housing. It was such a small leak that the coolant was boiling off immediately making it tough to find. eventually the system was losing nearly a litre in seven miles.