THERE’S a reason passengers might hear a “barking” noise during a long flight – and it’s got nothing to do with dogs.

During a flight an aeroplane might make various strange noises which might unsettle nervous fliers but invariably they are nothing to worry about.

The sounds are just part of the aircraft’s normal operation and knowing about them can help ease any concerns a flier might have and help them relax and enjoy the flight.

The “barking” you might hear is unlikely to be coming from a real dog but rather a mechanical barking sound that has an element of whirring in it.

This sound is specific to Airbus planes, according to Travel and Leisure.

Former pilot Dan Bubb, a professor at the University of Nevada, Las Vegas, said: "That barking sound on Airbus aircraft comes from the Power Transfer Unit.

Pilot Patrick Smith explained in his book Cockpit Confidential why it was only Airbus aircraft that made this noise, writing: "Boeing aircraft also employ a PTU, but the operation is slightly different and it doesn’t bark like a dog.”

There are some other noises you might want to familiarise yourself with so you don’t become unnecessarily worried.

Whirring

A whirring sound can invariably heard before take-off – this is just the plane’s flaps – the panels on the wings – being extended.

This modifies the shape of the wing to give the aeroplane more lift at lower speeds.

If you look at the wings when you can here the noise you’ll be able to see the flaps extending.

You’ll hear the whirring sound once again after take-off as the flaps are retracted, and once more as they are extended shorty before landing.

Bumps during take-off

As the plane gathers speed down the runway, you might hear and feel a number of rhythmic bumps.

This is caused by the nose wheel going over a series of lights called centreline lights, which run down the centre of the runway.

This is a good thing as it means the plane is perfectly centred on the runway.

The noise will stop as soon as the nose lifts and the wheels leave the ground.

A rumble and thud during take-off

Former flight attendant Sue Fogwell said: "The first major sound after take-off is the landing gear.

“Passengers will hear it retracting and closing with a loud sound.

Firstly, you’ll hear a mechanical whirring as the wheels are retracted into their wells.

This will be followed by a loud thud or two as the bay doors are closed.

Bubb said: "When it is retracted, you might hear the motor, but you will feel a series of rattles.

”Those are the snubbers, which are like brakes that stop the spinning of the tires when they are in the wheel wells."

‘Powering Down’ after take-off

The next noise you might hear will be what sounds like the engines reducing their power.

And that’s exactly what it is but there’s no need to worry that the engines have entirely lost power.

Smith said in his book: "The amount of thrust used for takeoff is always more than enough, so typically at a thousand feet or so, depending on the profile, it’s brought back to what we call 'climb power’.

"This saves wear and tear on the engines and keeps the plane from exceeding low-altitude speed restrictions.

“The plane is neither descending nor decelerating; it’s just not climbing as rapidly."

Dings

A series of dings or chimes are likely to be heard during the flight and these can be in different patterns or tones.

This is just the crew communicating with one another, or even the passengers.

Fogwell said: "After take-off, once the plane levels off, and it's at a comfortable angle, passengers will hear a double ding.

"It's to notify flight attendants that they can get up from their jump seats and begin their inflight duties."

A single ding will be heard to accompany the fasten-seatbelt light, too, telling passengers that it's either safe to move about or that you need to buckle up.

There can also be dings to alert flight attendants to take their seats or dings to ask flight attendants to pick up the phone.

Each airline has its own specific sounding digs so you might hear different tones or patterns on different flights.

A thud and a rumble during landing

The sound of the landing gear will be heard in reverse as the plane comes into landa as the landing gear will need to be extended before touching down.

Bubb said: "When the landing gear is extended, you will feel a thump and hear a loud rush of air.”

There’s nothing to be worried about with the sound of the rushing air as this is just the drag created by the landing gear.

What should you do if you are worried about an airplane noise?

While there is nothing to be worried about most sounds on an aeroplane, it is a good idea to speak up if you do have any particular concerns.

Fogwell said: "If a passenger sees or hears something alarming or unusual, immediately tell a flight attendant.

"Flight attendants rely on passengers in many instances."

She added: "A passenger can also ask the flight attendant to ask the pilot a question when they're not busy.

"When at cruising altitude, especially on longer flights, pilots don't mind a flight attendant asking a question for a passenger, as long as it's a non-security-related question.”