Before your plane takes off and after it's in the air, you'll hear a whirring sound as the flaps first increase the wingspan and then retract to help give lift, so the plane can take off and maintain air speed. Again, totally normal.

The "barking," "sawing," or "grinding" sound you often hear before takeoff—especially on Airbus A320 aircraft—is the Power Transfer Unit (PTU). This device is a hydraulic pump that ensures both hydraulic systems on the plane have equal pressure. It activates when only one engine is running (common during fuel-saving taxiing), creating a loud, rhythmic mechanical noise that many passengers mistake for a dog barking or a saw cutting wood. Other common sounds include the high-pitched whine of the Auxiliary Power Unit (APU) at the back of the plane, which provides electricity and air conditioning while at the gate, and the "thud" of the cargo doors being locked. You may also hear the "bings and bongs" of the intercom system, which are often coded signals between the pilots and cabin crew indicating that the cabin is "secure for takeoff."