No, the noise is still a mystery. As the car appears to drive completely normally apart from the noise when the engine is cold, I have decided to wait it out until whatever it is develops, as it surely will, and manifests itself in a way that points to the fault. I just hope it doesn't suddenly break down in an awkward place!
I will post the finding if and when I get to the bottom of it. In the meantime any other suggestions on what is causing my beloved Crossfire to blow like a flute when cold would be much appreciated.