Right - red herring. Dizzy back on and no difference. Sound definitely appears to be coming from the dizzy (that's the rear for me Wilf
) end.
It's back to the air/vacuum leak theory. I think I'm at the stage now where I need to remove the inlet manifold. I wish I'd planned ahead and ordered a new gasket today .....
The curious thing is that my vacuum gauge (which is plumbed into the rear half of the inlet manifold) is showing normal idle vacuum, and the engine idles fine without any hunting - surely a vacuum leak which results in such a loud noise would cause a weak mixture which would be evident at idle?