The stat will not be hissing. Its possible that thats the coolant filler cap as you say and the spring is lifting slightly as it is exceeding max op pressure. If that is the case then you need to investigate why it is exceeding. Do you have any other signs of overheating? I would suggest you go the "D2 - Common Faults-Tech Dump" thread and pull up the appropriate service manual to identify the plug.
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2002 D2 S8 – Ming Blue, Valcona leather, Vavona wood insert, solar sunroof (to be fitted), Heated rear seats, extended leather pack, 18” Avus, ski hatch, Bose, auto dim rear view mirror, rear blind.
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