Aussie Dave, you mention you have a hitchcocks pipe and shield, are you also running the Hitchcock's EFI bottle silencer? I run one of these, with a fishtail stuck on the end. The bolt-in baffle has a habit of working loose, and when it does it gives off a fiendish rattle on the over-run. Perhaps you could revert to the original exhaust system, which would rule out (or rule in) the silencer and baffle as a source of your noise?
Cheers,
Blairio