i had this same problem before christmas, i was sitting at a set of lights and i heard whistleing/gurgle noise, i looked out the rear window and seen steem from the engine bay just where the filler cap is, i pulled over and there was coolant all over the place, luckily i was only 2 seconds from my work so i hobbled there and got the car on the ramp, same situation bled the system and all seemed well apart from the coolant guage not reading but after about 45 mins it burst into life, but hasnt quite read right since.
then last week i just came off the motorway and looked in the rear view to find steem once again.
this time when i checked the car i found a coolant leak whch maybe coming frome the thermostat housing or further up at the HG, cant be sure form just a quick look.
but my coolant pipes are now pressureised to the point there about to burst and this is only at idle, its a horrible feeling mate but im almost positive its a HG, so its time for the engine to come out and get stuck in.
was getting the engine out after the summer for uprated parts and an overhaul anyway, just inconvenent and bad timing.
i wount drive the car mate untill your 100% its sorted, best of luck
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