Strange one tonight, stuck in traffic on A406 near Neasden in London for few good minutes and as I was moving sloooooooooooowly heard some funny noise, similar to a helicopter noise really far in the sky. Lowered the window and no sign of a helicopter, put the windows up again and noise came back. All happening in less than a minute. Then I checked the dashboard and the water gauge was on red, stopped immediately just off A406 and opened the bonnet. The coolant in the reservoir was boiling really hard and as soon as I stopped the coolant was coming off the overflow, must have been at least 500ml-1litre spilled in the road.
Being afraid that it could be the head-gasket from overheating I called RAC, didn't want to start the car again. An hour later, the guy came, add some water into the radiator directly and checked all hoses, all dry, no leaks. Started the engine, the engine held its pressure, radiators kicked in in few minutes, water level bellow half way on the temperature gauge, engine absolutely fine! Drove home for another 15 miles with no problem at all, oil level fine, coolant fine, no overheating.
The RAC guy said it could be a malfunctioning thermostat and that I should have it checked tomorrow, adding that it was very very odd.
What do you think it might have been?