Noise at back door

[rotjohns]

Something I've always wanted to know. In the older transit buses (TMC CTA buses for example), when the driver lights up the green light for the back door, then shuts it off, you always hear a noise. When I rode other buses in other cities, their older buses did it too. I've always wanted to know what that noise was and if it was any significance? In some cases, it happens even when the front door closed (old Indiana University buses come to mind with these). Even if you get on Pace's Eldorado buses, when the front door closes, you hear a hissing noise, then the bus takes off. Just was curious. It's hard for me to describe the noise, but I'm sure veteran CTA drivers know exactly what I am talking about.

[berthapegues]

Something I've always wanted to know. In the older transit buses (TMC CTA buses for example), when the driver lights up the green light for the back door, then shuts it off, you always hear a noise. When I rode other buses in other cities, their older buses did it too. I've always wanted to know what that noise was and if it was any significance? In some cases, it happens even when the front door closed (old Indiana University buses come to mind with these). Even if you get on Pace's Eldorado buses, when the front door closes, you hear a hissing noise, then the bus takes off. Just was curious. It's hard for me to describe the noise, but I'm sure veteran CTA drivers know exactly what I am talking about.

That hissing noise you hear is the air pressure that is used to open/close doors. Air pressure is used for other items on buses as well such as brake.