Why Do Trucks and Buses Make Hissing Sounds When Brakes Are Applied?

Trucks, buses, and other large vehicles often make loud hissing or air release sounds when the brakes are applied. This sound is normal and is caused by the vehicle’s air brake system releasing compressed air. Understanding what causes this sound can help drivers and passengers know what to expect when riding in or operating these large vehicles.

Here’s a quick answer: Trucks and buses make loud hissing sounds when the brakes are applied due to compressed air being rapidly released from the brake system. When the brakes are activated, air pressure builds up and is then discharged through valves and hoses which creates an audible hissing noise. This sound is completely normal and indicates the air brakes are functioning properly.

How Air Brakes Work on Trucks and Buses

Most modern trucks, buses, and other heavy vehicles are equipped with air brake systems. Air brakes use compressed air to activate the brake pads and slow the vehicle.

An air compressor on the vehicle’s engine builds up and stores compressed air in reservoirs. The compressed air travels through hoses and valves to the brake chambers at each wheel. When the driver presses the brake pedal, the valves open to allow compressed air into the brake chambers.

The compressed air pushes a diaphragm and brake pads against the surface of the brake drum. This friction is what slows the wheels of the vehicle.

As compressed air enters the chambers, it also releases the previous air pressure back through valves and out through the hoses. This release of pressurized air makes the loud hissing noise.

Why Air Brakes Make Hissing Noises

There are two main reasons why air brake systems cause a hissing sound when applied:

1. Air Compression Release – As noted above, applying the brakes releases the previously compressed air from the lines and chambers. This rapid release of pressurized air through valves and small openings creates a loud hissing noise. The larger the vehicle and more air volume released, the louder the sound.

2. Air Dryer Purge – Air brake systems use air dryers to remove moisture from the compressed air. When the brakes are applied, the air dryer also purges the collected moisture through valves – which contributes to the hissing sound.

Essentially, the hissing comes from pressurized air molecules moving rapidly through small openings all at once. This quick release of compressed air is loud enough to be heard inside and outside of the vehicle.

Why the Hissing Sound is Normal

The hissing or air release sound when applying air brakes is completely normal and an important part of how the system functions. The sound indicates that the brakes are functioning properly by releasing built-up air pressure as intended in the design. The loud hissing noise does not imply any air leak in the vehicle, as the air is being purposefully discharged through the release valves in a controlled manner.

Furthermore, the volume of the hissing provides confirmation that the air dryer is operating effectively to purge moisture from the system. The rapid movement of a large quantity of compressed air through the brake lines and chambers makes some audible noise inevitable upon release. In older air brake systems, the sound was even louder until mufflers were added to reduce the volume.

Therefore, while the hissing may sound startling initially to some riders, it is a standard result of the air brake depressurization process. The sound simply signals that the brakes are operating as they are designed to do.

Tips for Passengers About the Hissing Sound

For those not familiar with large vehicles and air brake systems, the loud hissing sound can come as a surprise. Here are some tips for helping passengers know what to expect:

  • Explain that the sound is normal and means the brakes are working properly. Knowing what causes the sound can prevent unnecessary alarm.
  • Find ways to limit the volume, such as keeping windows closed or wearing earplugs. While loud, the sound itself is not dangerous.
  • Get used to the sound over time. The more you ride in vehicles with air brakes, the more the sound just becomes part of the background.
  • Focus on other things rather than the sound, such as enjoying the view or talking with other passengers.
  • If the volume seems excessive, mention it to the driver as the system may need air line mufflers or other repairs. But expect some audible hissing each time.

Design Factors That Reduce Hissing Volume

While a certain level of hissing sound will always be present, vehicle engineers use various design factors to limit the volume:

  • Mufflers – Special mufflers or silencers are installed on the air lines and valves. These are tuned to allow pressurized air release while reducing noise.
  • Hose Length – Keeping air release hoses as short and direct as possible between system components reduces sound reverberation.
  • Number of Release Points – Having fewer places for air to escape means less cumulative sound. But enough release points must be designed for proper system function.
  • Release Valve Angle – Orienting the air exhaust valves away from passengers directs the sound away from the cabin area. Angled valves also diffuse the noise.
  • Cabin Insulation – Major manufacturers use extra sound-dampening insulation and materials in the vehicle cabin to further reduce the perceived hiss volume.

While these design factors help, they can only limit the loudness so much. Some audible hissing will always be present when applying air brakes, but it does not indicate any problem with normal operation.

Other Noises Truck Air Brakes Can Make

The primary sound air brakes make is a loud hiss when releasing compressed air. However, some other sounds can occur related to the air system:

  • Squeaking – Dry or worn brake chamber diaphragms can cause a high-pitched squeaking noise when the brakes are applied. This indicates worn parts need replacement.
  • Clunking – Loose or worn linkage between the brake treadle and valve can cause a clunking sound from the impact. Linkage should be inspected and tightened.
  • Whistling – Small air leaks in hoses, valves, or fittings will create an audible whistling sound. Leaks need to be located and sealed.
  • Banging – A loose air compressor, mounting bracket, or discharged air line can create a loud banging sound. Secure mounts should be installed.

So while a certain level of hissing is normal, other noises like squeaking or banging may indicate maintenance is needed. Being familiar with these sounds can help identify issues.


The loud hissing or releasing air sound when applying air brakes on trucks and buses is a normal function. It may be startling for those not accustomed to large vehicle air brake systems, but the sound itself is not a cause for concern.

Knowing the source of the sound can prevent unnecessary worry. While some volume is unavoidable, vehicle designers work to minimize the noise level where possible. Over time, riders get used to the brief loud hisses as just part of riding in buses, tractor trailers, and other heavy vehicles equipped with air brake systems. Just focus on the sights or conversations instead, and enjoy the ride!

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