For everyone from the US saying you thought the sound was air brakes, it probably was. I had to look up video of an actual truck with the white noise alarm (since I've never heard on in person here in the US), and the alarm definitely sounds different from air brakes imo. Air brakes have that initial plosive phase to the sound (an exponential decay if you will), where as the white noise alarms are gated like a square wave (either 100% volume or 0% volume). Plus you still hear the beep type alarm at construction sites all over the US, so I don't think the new type has been adopted here yet.