Driving through the Dordogne (as you do!) a couple of weeks ago in temp of 30C a squawk developed which got progressively worse. Visions of having to abandon car in France passed through my mind. Eventually I stopped and with great trepidation, opened the bonnet. A colourless fluid streaked the heating pipes. I tasted it - water? There was no loss of coolant or brake fluid. It dawned on me that it could be condensate from the aircon system. I had been driving with the recycle fan off and the vectra had been trying to cool the whole climate of France. The condensate had been swept back into the engine compartment instead of dripping on the road. I reset the fan to recycle and the squawk diminished and never returned. I can only think that the noise came from wet rubber belts.
Anyone got another explanation.