Turns out, there’s no doggie in the window.

You’re bound to hear a few whirring noises and loud thuds on your flight, particularly during severe turbulence or as the plane takes off and lands. Even for experienced travelers and frequent flyers, a sudden screech or jarring explosion of sound can turn a seemingly comfortable flight into a scary nightmare in the friendly skies. But you shouldn’t panic and reach for your aircraft safety card every single time you hear things that go bump on the flight.

In fact, most noises are completely safe and routine, like the contents inside the overhead bins or galleys that cause a little ruckus when the plane departs or jolts midair. But there’s one strange noise that keeps passengers on edge, and it’s what aviation experts and pilots call "the barking dog" because it sounds like a low growl or barking. Does it mean the wheels are falling off or the engine is failing? Should you be alarmed? In short, no. The "woof" sound effect, surprisingly, is a good thing—although probably not to your ears.

According to Patrick Smith, airline pilot and writer of Ask the Pilot column, the noise can be heard on specific Airbus plane models. These twin-engine models feature a device called the power transfer unit (PTU), which, in layman’s terms, helps to conserve fuel. It also is designed to power the hydraulic systems in the aircraft. As the PTU cycles on and off when pressure fluctuates, it generates a loud, recurrent sound like that of a dog barking. You can expect to encounter this noise a lot, especially since American Airlines, British Airways, Delta, Frontier, Spirit, and United Airlines all use Airbus models with twin engines and PTUs.

Next time you hear this barking noise, don’t fret. You can relax knowing the engine is functioning as it should to ensure that you arrive to your destination safe and sound.