Green’s Dictionary of Slang

the bell (later replaced by a siren) on a police car or similar vehicle; also attrib.; thus gong beater, the mechanism that produces the sound.

1906		A. Train Prisoner at the Bar 106: They bundle Maria unceremoniously inside, escort her erstwhile employer [...] into the same vehicle, [...] the gong rings: and Mr. Appleboy starts upon his task of bringing an evil-doer to justice.
1922		S. Lewis Babbitt (1974) 174: A Black Maria – gong beating like a terrified heart.
1930		J. Lait Put on the Spot 4: A few yards behind, its gong beater banging, sped the patrol wagon.
1937		J. Curtis You’re in the Racket, Too 129: Will [the police] be coming in a car with a syren like they do in the pictures [...] Of course, they only have those silly gongs in this country.
1943		R. Llewellyn None But the Lonely Heart 284: ‘It’s a gong job, I tell you,’ Taz says [...] right behind them, the queer, weak, ringing of a police car.
1947		S. Lewis Kingsblood Royal (2001) 320: The gong was heard, the patrol wagon pushed politely through the retreating crowd.