If your vehicle is showing hissing or whooshing sound of rushing air under the hood, here are the most likely causes and the parts you'll need to fix each one.
A disconnected, cracked, or improperly sealed intake hose or vacuum line allows unmetered air to be sucked into the engine. This is a very common error after replacing an air filter or performing engine maintenance.
The Positive Crankcase Ventilation (PCV) system uses engine vacuum to draw out crankcase gases. A cracked or disconnected PCV hose or valve will create a loud, constant hissing noise at idle.
The brake booster relies on engine vacuum to assist with braking power. If the internal diaphragm fails or the check valve cracks, it will hiss loudly, especially when the brake pedal is pressed.
On turbocharged vehicles, pressurized air can escape through loose intercooler pipe clamps, a cracked inlet pipe, or a stuck blow-off valve. This creates a loud whooshing sound under acceleration and is a frequent installation error after aftermarket modifications.
A pinhole leak in a hot radiator hose or a failing radiator cap can release highly pressurized steam. This sounds exactly like a loud hiss and is usually accompanied by a sweet smell and visible vapor.
If an A/C line ruptures or the pressure relief valve triggers, high-pressure refrigerant gas will violently escape. This causes a very loud, temporary hissing sound followed by a loss of air conditioning.