Hi Rory I have the same sound / siren . I took my brother out west to Cowra NSW and when he heard the noise he thought to himself that the police will catch us soon enough. Needless to say they didn,t . Noise started one morning and has been with me for some 15,000km. No smoke, no power reduction, no rattle , no nothing except a few turning heads .Took to local mechanic and he checked it out . Couldn,t find or suggest any cure. Spoke to the bro who has had a mechanical business for 40 years and his immediate suggestion was turbo noise. Suggestion is live with it .
