Head Gasket?

Hi All,

AUQ 130k miles

Has been losing coolant and white smoke out of exhaust for a while

Replaced some coolant pipework and thermostat which stopped it for the most part where it appeared to be leaking

Notice a slight shimmering film on top of coolant reservoir before driving

Drove for several hours and coolant red light on dash came on with alarm

As close to home (and temperature not going up) drove a mile or two to house and parked up

Left overnight and checked coolant this morning and its changed colour to a murky brown (from darkish pink)

The level of coolant it above minimum so not sure why the alarm went off as level and temperature seemed fine.

One thing I noticed when opening expansion tank was a pop and hiss (escape of gas?) which hasnt happend before especially after leaving over 12 hours.

Is this likely to be a head gasket gone or an oil cooler letting oil into the coolant or something else?

no white may on dipstick or under oil filler cap (does use a bit of oil though as need to keep topping up)

Any advise appreciated,

Thanks