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Sounds like the rad cap letting off steam. Whats the temp gauge say when it makes the noise?. My k10 gauge stays rock solid a 1/2 even if i rag it about. If i get stuck in traffic it will go up a tiny bit then the fan cuts in and takes it back to 1/2. But back to the piont try letting the car sit at idle with the bonnet up and try to hear where the noise is comming from. The wifes k11 had a hissing sound when her heater matrix was leaking but it was a constant hiss when warmed up.....