Bus noise

[Mudit]

Why does a bus make a hissing noise when it comes to a stop at a bus stop?

[wildwood]

Probably the air-brakes.

[sddsddean]

As wildwood says, buses, like trucks have air brakes. The air brake chamber(one to each wheel) has a massive spring inside it which is trying to put the brakes on. This is a fail safe. If the air supply goes, the vehicle should stop. So the air acts on a piston which keeps tthe spring compressed and the brakes off. So when you slow down, as you apply the brake a bit of air leaks out. When you come to a stop, you apply the park brake which lets all the air out and it goes 'Tssschh' (or something similar!).