To understand a hissing sound near the brake pedal, compare it under normal, safe conditions: cold versus warm, idle versus steady speed, and light versus higher load. Note whether it occurs only while pressing the pedal and whether pedal effort has changed. Change one condition at a time so the result stays useful.
Do not create a risky test just to reproduce it. Check for a change in brake assist; do not try to repair vacuum components without service information.
Quick educational note
For information, not a diagnosis or repair instruction. Vehicle-specific checks are still required. Consult the manufacturer's guidance and a qualified technician.
How to use this information
This article explains general principles and does not establish what is wrong with your vehicle. Read the content limitations.