If you’ve flown on an Airbus narrowbody (A318/A319/A320/A321) or an A330, you’ve definitely heard the sound that makes you think something is horribly wrong, especially if you’re sitting near the wing. As you wait to take off, you look out the window expecting to see this:

But don’t worry, it’s not actually a ninja repeatedly kicking a seal in the head, nor is it a dog that got loose in the belly of the plane . . . it’s just the Power Transfer Unit (PTU).

First, here’s a little background. These aircraft each have two separate hydraulic systems. Those hydraulic systems power some pretty important things, like flight controls. Each of those systems is powered by an engine-driven pump. One is on the left side while the other is on the right.

When both pumps are working, there’s no need for the PTU, but when only one pump is working, the PTU enables a single pump to also power the hydraulics on the opposite side. It’s most common to hear this while taxiing, because you’ll often taxi with one engine to save fuel. When both engines are running, all sounds normal.

So next time you hear the noise, just remember it’s nothing bad. Now you can reassure your fellow passengers that all is well. (Cue “The More You Know” music.)

For more info, read this article.