A pilot explains the sonic world of commercial flight in an attempt to reassure nervous flyers.

There are any number of strange noises you hear during a flight that can unsettle even the most assured of passengers.

It might be a clang that jostles you from your reverie or a whirr that makes you tightly grip your armrest. Maybe it's a horrible thud — the kind that prompts you to search for signs of panic among the flight crew — or even just a normal-sounding ding that remains a total mystery.

Because the unknown can be scary, especially when you're hurtling through the sky, The Smithsonian has asked Captain John Cox, a pilot with more than 14,000 flying hours under his belt, to demystify the sounds you'll hear before and during a flight.

All of them are routine but there's a rarer one in particular that usually scares the bejesus out of everyone on board.

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Before take-off: Chimes

You'll notice a lot of chimes before takeoff. These are intercom calls.

One ding indicates a call from the cockpit to the cabin crew, and two dings indicate that a member of the cabin crew wants to chat to another.

Before take-off: Pounding from underneath the plane

About 10 minutes before takeoff, this sound — followed by two whirring sounds — is the cargo hold door is being closed.

Before take-off: Sound of heavy wind as plane begins to taxi

When the plane starts moving on the runway to prepare for takeoff, this sound indicates a change in the air source.

"Air is used to start the engines and to cool and heat the cabin. Therefore, it must be redirected," Cox tells The Smithsonian.

"Wind noise can indicate that the redirection is taking place."

During take-off: Two loud thuds

These double thuds, coming from under the plane, is the sound of the landing gear retracting. The thuds are followed by a big whooshing noise, kind of like a spinning propeller — this is the sound of engine air changing as the engine's RPM increases.

Most of the unusual noises you hear in-flight can be easily explained. Photo / 123RF
Most of the unusual noises you hear in-flight can be easily explained. Photo / 123RF

About five minutes after take-off: Two loud dings

You know these "dings" — we use them as our cue to start using electronic devices again or that we're clear to nip off to the toilet.

But they serve a more technical purpose: they actually indicate the plane has reached 10,000 feet, or roughly 3050 metres.

This is a crucial milestone, especially for planes flying under the regulations of the US Federal Aviation Administration. Anything under 10,000 feet is considered a critical phase of the flight and under FFA rules the flight deck can't be contacted about non-essential issues.

So once the two dings sound out, the cabin crew know they're now free to contact the flight deck.

During flight: Rattling and creaking

You may hear a lot of rattling and creaking from different components of the aircraft while the plane is taxiing or in the air.

These aren't pleasant sounds but they're perfectly normal, Cox says. There's a lot of room for components to move, especially in the galley. Think of it as being similar to a house settling.

During flight: Even more chimes

Chimes during the middle part of the flight are more intercom calls between crew members, just as before takeoff.

Often they hint that food and drinks are on their way as the cabin crew discuss the upcoming service.

On descent: Two loud dings, about five minutes before landing

It's the reversal of the other two dings you hear after take-off. As the plane approaches its destination, the two dings sound again to indicate the aircraft has descended down to 10,000 feet and is entering the next critical flight phase.

It's essentially code from the pilot and co-pilot for: "Don't bother us".

The 'dings' you hear about five minutes after take-off indicate the plane has reached 10,000 feet. Photo / 123RF
The 'dings' you hear about five minutes after take-off indicate the plane has reached 10,000 feet. Photo / 123RF

On descent: High-pitched whirring

About five minutes before landing, you'll hear the sound of the slats and flaps on the aircraft being extended on the wings so the plane can land, as well as the hydraulic motor that controls them.

Right before landing: Big thud

This is the reassuring sound of the landing gear emerging as the plane approaches the runway.

Any time during flight: The big sound that scares everyone (see video below)

This video shows the moment a compressor stall occurred on an American Airlines flight during take-off. The plane, which was due to fly from Chicago's O'Hare International Airport to McCarran International Airport in Nevada, returned safely to Chicago shortly after the incident. Source: Gabe Gatmaitan, via YouTube

There is another, rarer sound passengers may hear that tends to make them scream and panic, Cox says.

It's a sound similar to a gunshot blast and the plane lurches with it. It can happen once or multiple times in a row.

This is a compressor stall and it's caused by a disruption to the airflow in one of the engines. And it's pretty much the sound you'd least like to hear on a flight.

Cox says it's not as bad as it sounds, because even if an engine stalls and fails, the plane can still fly with its remaining engines.

The worst case scenario is all the engines stalling, which is what happened to US Airways' twin-engine Airbus A320 that ditched in the Hudson River in New York City in 2009.

So what sound will you hear if there really is something to worry about? The sound of a flight attendant telling you to assume the bracing position is probably your surest bet that something may — but not always — be wrong.