That hissing is too much a compressor overflow discharge. That's exactly what it sounds like. We hear it all the time. When an air tank gets overfilled, the safety valve lifts, it goes 'hiss', the valve closes at the safe pressure, but if the cylinder is still charging, the whole lot happens again and again. That is exactly the sound of escaping air. As lorries have air brakes and none of us really bother about what the mechanical gubbins are about, all we hear is escaping air from an overcharging air tank, and simply ignore it. You'll get the same sound on trams and buses with air operated doors and brakes. There'll be an obvious compressor noise too, but not always cos hey, modern technology, a quiet compressor, so all we hear is escaping air and that's it.<br>I've NEVER EVER associated it with reversing vehicles. It sounds too much like escaping air which you do not associate with reversing or moving anything.,(reply>), It doesn't sound anything like that IRL though.