Hi D
Wait until the engine is cold to the touch then check and top up the radiator with water, leave the cap off and start the car up turn heater control to hottest setting, fan set to low speed. Check continuosly to top up with water, as the engine warms a little the level will tend to fluctuate a little. After it has settled a little have a friend hold the car at around 1500rpm and observe the water in the radiator, observe to see if any bubble show up, if so have your local check the cylinder head for compression leakage into the cooling system.
The hissing sound is not good, If you cannot get the car sorted then it'll be worth your while to travel with a friend as overheating can be terminal !
Once problem sorted fflush and fill your cooling system with correct mix of antifreeze and water as this also reduces corrosion. Be careful with any left over antifreeze as it's very toxic or fatal to dogs.
Regards Don
New Zealand
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Wednesday, January 20th, 2010 AT 5:57 AM