It wouldn't work on steam yesterday. So today I had a brainwave, try it on compressed air (car tyre) and at a low rate it sounded fine but then rapidly blew out as the supply was opened up. It needed a restrictor. A piece of 1/16th tube was placed inside the whistle's 3/32" inlet tube and it worked at all flow rates. The steam test has the restrictor fitted. The car tyre is at about boiler pressure - 30 psi, it just needs pumping up afterwards...

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