Did anyone else just think it was a rush of escaping air from an air-brake system in a lorry or such?<br><br>The advantage of a single tone is it's not found anywhere else in the world, so it's easily distinguishable.,(reply>), @echodelta9 air brakes hiss loudest when air is evacuated from the chambers. This means that the driver has set the brakes so the truck won’t roll. They do hiss when the brakes are released, and the brake chambers fill with air, but unless you are standing right next to the truck, you won’t hear it, and it is hard to hear over the trucks engine. Not to mention that it is a very different sounding hiss.,(reply>), Just cut the usual "This vehicle IS...." soundbite short and leave passersby guessing.,(reply>), @LegoVogel Except air brakes in heavy vehicles (especially trailers) are release to apply.,(reply>), @Cyber Dragon Zekrom<br>“Leaping to unsupported conclusions in a single bound ...”<br><br>Should have minded the sign that says not to feed the trolls. I’m making a sandwich. Bye.,(reply>), @Kurtis Kroon Then you're getting people killed for no reason than to justify your employment.