system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5 : why do buses make that "PFSSSST" sound when they stop?
gpt
Many large vehicles have a system of brakes actuated by compressed air instead of hydraulic fluid or cables you would find in a lighter vehicle or passenger car.

Parking/emergency brakes on such vehicles are usually "normally closed": to release the brake, air pressure must be applied to the brake.  For these, a sudden release of air pressure will quickly activate that brake, but also will create a little bit of noise.