Whether you’re a first-time flier, a nervous flier, or a curious frequent flier, you might have questions about all the noises you hear during a flight. And no, we don’t mean babies crying or the jarringly loud drop of an iPhone.

Airplanes are large, complicated machines. During the flight, they make a series of noises that some passengers might find unsettling since they’re not something you hear every day (unless you’re a pilot or flight attendant, of course). These sounds are typically a normal part of the aircraft’s operation, and understanding what they are and why they occur can help ease any concerns and make your in-flight experience a little more comfortable.

Whirring

If you hear whirring before takeoff, that’s the aircraft’s flaps — essentially panels on the wings — extending. This modifies the shape of the wing, creating more lift at lower speeds (in other words, the plane can take off more easily). You can actually see the flaps extending if you look at the wing when you hear this noise. They’ll whir again after takeoff as they retract, then again as they extend before landing.

Rhythmic Bumps During Takeoff

As your plane is accelerating down the runway, you might hear and feel a series of rhythmic bumps: thump thump, thump thump, thump thump. No, the wheel isn’t loose and about to fall off, but it is the wheel making that noise. Along the center of the runway are a series of lights called (appropriately) centerline lights. And the bumps you hear are the nose wheel rolling over those lights. This is not only normal, but a good thing, as it indicates your plane is perfectly centered on the runway. You’ll notice that these bumps stop as soon as the nose lifts up and the wheels leave the ground.

A Rumble and a Thud During Takeoff

“The first major sound after take-off is the landing gear. Passengers will hear it retracting and closing with a loud sound,” says Sue Fogwell, a former flight attendant. First, you’ll hear a mechanical whirring as the wheels are tucked into their wells, then you’ll hear a loud thud or two as the bay doors close. “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,” says former pilot Dan Bubb, a professor at the University of Nevada, Las Vegas.

Engines “Powering Down” After Takeoff

Next up is what sounds like the engines reducing their power — and that’s exactly what’s happening. But it’s not because they’ve lost power entirely. “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,'” writes pilot Patrick Smith in his book Cockpit Confidential. “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

During the flight, you’ll likely hear a series of dings or chimes, often in different patterns or tones. This is the crew communicating with each other — or in some cases, with the passengers. “After take-off, once the plane levels off, and it’s at a comfortable angle, passengers will hear a double ding,” says Fogwell. “It’s to notify flight attendants that they can get up from their jump seats and begin their inflight duties.” A single ding will accompany the fasten-seatbelt light, too, alerting passengers that it’s either safe to move about the cabin or that you need to buckle up. There may also be dings that alert flight attendants to take their seats, or dings to ask flight attendants to pick up the phone. Every airline has its own “language” of dings, so you might hear different tones or patterns on different flights.

“Barking”

While there may actually be a barking dog on your plane, we’re talking about a more mechanical barking sound that has a little bit of whirring in it. This is a very specific sound to Airbus aircraft. “That barking sound on Airbus aircraft comes from the Power Transfer Unit. It transfers hydraulic power from one system to the other once the pressure reaches 500 PSI,” says Bubb. Why do you only hear this on Airbus aircraft? “Boeing aircraft also employ a PTU, but the operation is slightly different and it doesn’t bark like a dog,” writes Smith.

A Thud and a Rumble During Landing

The sound of landing gear retracting plays out in reverse later on in the flight, as the landing gear needs to be extended for you to land. “When the landing gear is extended, you will feel a thump and hear a loud rush of air,” says Bubb. Don’t be too worried about the sound of rushing air, as it’s simply the drag created by the landing gear. (That’s why we retract landing gear during flight — to reduce drag).

What should you do if you’re worried about an airplane sound?

Most sounds on an airplane are nothing to be afraid of, but if you have any concerns, it’s always a good idea to speak up. “If a passenger sees or hears something alarming or unusual, immediately tell a flight attendant,” says Fogwell. “Flight attendants rely on passengers in many instances.” But Fogwell also notes that you can simply ask flight attendants — or even pilots — any questions you have to make you feel more comfortable in flight. “A passenger can also ask the flight attendant to ask the pilot a question when they’re not busy,” she says. “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.