edit: Solved. Thanks guys
In: Engineering
It’s the brakes.
Cars like yours or mine use hydraulic braking fluid to compress the brakes. This isn’t loud enough to provide a distinctive sound (unless something is wrong with said brakes).
Trucks and busses used compressed air. This air needs to be kept at the right pressure for the brakes to work properly, and the use of the brakes causes the pressure to increase, so a valve needs to release pressure to keep that in check. The hissing sound you hear is that mechanism at work, releasing the air.
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