If your vehicle is showing hissing sound coming from the dashboard, here are the most likely causes and the parts you'll need to fix each one.
The A/C expansion valve regulates refrigerant flow into the evaporator inside the dash. If the system is low on refrigerant or the valve is failing, it creates a distinct hissing or sighing noise when the A/C compressor engages.
The brake booster uses engine vacuum to assist braking. A torn internal diaphragm or a leaking pushrod seal near the brake pedal will cause a continuous hiss that usually changes pitch or stops when you press the brakes.
Many vehicles use engine vacuum to control the air vent mode doors. A cracked vacuum line, disconnected hose, or failing control switch behind the dash will hiss, often changing when you adjust the climate controls.
Trapped air bubbles or a pinhole leak in the heater core can cause a hissing or gurgling sound as hot coolant circulates behind the dashboard. This frequently occurs if the cooling system wasn't properly bled after recent maintenance.