If your vehicle is showing hissing or vacuum sound from under the hood, here are the most likely causes and the parts you'll need to fix each one.
Rubber vacuum hoses and PCV lines become brittle and crack over time due to engine heat. They are also frequently knocked loose by accident during routine oil changes or air filter inspections, causing unmetered air to rush in and create a hissing sound.
If the air filter box isn't clamped down properly or the main intake tube is loose, the engine will suck in air through the gap. This creates a loud rushing or hissing noise and is a very common error after checking or replacing the engine air filter.
The gasket that seals the intake manifold to the engine block can degrade, shrink, or crack. This allows air to bypass the throttle body, creating a high-pitched hiss and usually causing a rough idle or check engine light.
The brake booster uses engine vacuum to assist your braking. If the vacuum check valve cracks or the internal diaphragm fails, you will hear a hiss near the driver's side firewall, and the brake pedal will likely become very hard to press.
A pinhole leak in a coolant hose or a failing radiator cap can spray pressurized, boiling coolant onto hot engine components. This creates a hissing sound from the steam, often accompanied by a sweet smell, especially after turning the engine off.
On turbocharged vehicles, the pressurized pipes connecting the turbo to the engine can blow off or crack. This results in a loud whooshing or hissing sound specifically when accelerating, often due to loose clamps after recent engine work.
If a high-pressure air conditioning line or the condenser is punctured by road debris, the pressurized refrigerant gas will rapidly escape. This causes a loud, temporary hissing sound until the system is empty, after which the A/C will blow warm.