In the late 90s, I flew CVG-BRU on a SABENA A340 and I was seating just in front of the wing. About an hour into the flight, this weird vibration was going through the plane. It got pretty loud and it was pretty much nonstop. After a while, the captain walks through the aisle, stops in front of the window and looks at the wing/engine for a couple of minutes. He then disappears back into the cockpit. The noise pretty much continued the entire flight and no announcements were made.