If it seems that the hissing noise is connected to braking, don’t ignore it and hope the sound just disappears. What Should You Do About a Hissing Noise? If the foam is missing or damaged, you may hear the hissing sound. Many vehicles that have brake boosters come with a piece of foam that blocks the hissing sound.

A leak of air from the master cylinder gasket into the brake booster.A problem with the booster diaphragm triggers the failure of the brake booster.A hole in the vacuum hose that works in conjunction with the brake booster.If something goes wrong with that process, it could lead to a hissing noise. This step creates vacuum pressure on one side of the diaphragm to support stopping power. When the pedal is depressed, a rod moves into the brake booster and master cylinder. Inside the brake booster is a diaphragm that controls the pressure when the brake pedal isn’t pushed down. The brake booster relies on vacuum pressure to do its job. But if you hear a hissing sound when pushing down or letting up on the brake pedal, it usually is caused by the brake booster leaking air, which could mean there’s a leak in the booster diaphragm, master cylinder gasket, or vacuum hose. You may hear a hissing noise if there’s a disconnected or leaking vacuum hose around the engine or an issue with your vehicle's serpentine belt. Fortunately, in most cases, it’s an issue that can easily be fixed! What’s Behind the Hissing Noise?Ĭars can make many noises, and a hissing noise when braking can be among them. But why are your car brakes making a hissing noise? Chances are, a mechanical issue is causing the hissing.

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