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4 Mysterious Cabin Sounds During Airplane Boarding Explained

(You may be surprised by the visual effect from the 4th sound)

Have you ever wondered what the cabin sounds are that you sometimes hear when you board your airplane?

I know it puzzles a lot of passengers because I get asked about it quite often!

Another question that follows the first one is whether or not something is wrong with the airplane.

Let me assure you, no, the sounds you hear are normal.

You may already feel nervous about flying as it is. And it is normal to feel nervous for many airline passengers.

As human beings, the fear of the unknown, things we aren’t educated about, sometimes scares us.

1. What is that intense humming sound?

That strange humming sound you may hear once you board your airplane cabin is often a passenger’s introduction to a series of harmless and different sounds you are going to hear throughout your entire flight experience.

The humming sound is a result of the Auxiliary Power Unit (APU) running.

APU inlet and exhaust on an A380
The visible parts of an Auxiliary Power Unit (APU) of an Airbus 380. The cylinder-shaped tube sticking out of the tail is the exhaust-part of the APU

What is an APU and what does it do?

An APU is a separate small jet engine located at the tail of large airplanes.

It is used to provide electrical power to the airplane when the engines are not running on the ground. The APU is also hooked up to the pneumatic system to power the air conditioning system.

You can compare it to a fuel-operated electrical generator you can buy at Home Depot.

access panel of an airplane apu
This APU engine is shown with the access panel open

Air powered from the APU can also be used to start the airplane’s engines. Furthermore, it can act as a back-up generator in the air, as a safety measure should the airplane’s generators malfunction.

If you didn’t have the APU as a back-up in the air, then only the battery would be able to provide electrical power should the airplane’s main generators malfunction.

A jet has one separate generator for each engine.

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As a matter of fact, on the old Boeing 727 aircraft, the APU was even designed to provide a small additional amount of thrust in flight as well! It did that by generating a small amount of thrust from the exhaust of the APU unit itself.

If you are interested in a basic explanation of how an airplane flies and what thrust is you can find out more by reading our article on the subject here.

2. What is that low-pitch continuous humming sound?

We prefer external electricity to come from the airport terminal powering the airplane on the ground. This is because the APU is running by jet fuel from the airplane’s fuel tanks, which in turn is expensive.

The power supply is hooked up to the airplane like it is on an RV or an electric car for that matter.

The electrical hookup produces a sound similar to that of plugging in your cordless electrical garden equipment into a power charger.

These power units are hooked up to the jetway, also called the jetbridge, leading to your airplane.

Jetway terminal electrical power
An airplane stationary Ground Power Unit (GPU)

The next best thing to external power hookup from the terminal is to have the airplane get its ground electricity from an external electrical power cart (“a portable electrical generator on wheels”), also called a GPU, or a Ground Power Unit.

The GPU, when supplying the airplane with electricity will also create that low-pitch continuous sound, just like the units located at jetways.

In addition, this cart produces outside noise like any other portable generator would do.

Airplane external power cart
An airplane portable Ground Power Unit (GPU)

If neither the jetway-connected GPU or the portable GPU is available, then the APU will be started to supply the airplane with electricity.

3. What is that whining Sound?

Another sound you may hear is a hollow-type whining sound. This sound, however, is not coming from the APU itself. Rather, the sound is produced by the air conditioning system’s Air Cycle Machine.

Airplane Air Cycle Machine

The Air Cycle Machine (ACM) is located in the belly of the airplane. Its purpose is to produce and process tempered air-conditioned air for the cabin, for the comfort of passengers and the crew.

On some jets, if you sit directly above the Air Cycle machine the sound may be more noticeable for you than it is for passengers seated further away from this location.

4. What is that hissing sound?

No, that hissing sound is not from snakes on your airplane.

Usage of the air conditioning system on the ground and in the air sometimes results in “hissing-sounding” fog blowing out of the cabin air diffusers in the cabin.

Fog can occur in the cabin when the temperature differences and moisture mix get to the point where fog occurs. This is only temporary.

Removal of the fog works similar to that of your car’s window fogging up inside. As soon as you turn on your defroster the fog disappears.

Have you been on a flight and seen that?

This too is completely normal and no cause for concern.

Regardless, here is a picture of what that looks like.

Fog in airplane cabin
Fog in the airplane cabin

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