Flying home from New Year in the Florida Keys, the second leg was overnight from Charlotte, North Carolina, into London's Heathrow. I was enjoying my champagne in business class, about 45 minutes into the flight, when there was a series of bangs from somewhere in the plane. The pilot came on to tell us that the crew could hear the sounds as well, and were turning the plane back rather than risk flying across the ocean with unexplained noises. It was a somewhat nerve-racking flight back because we didn't know if the wheels would come down. They did, and we landed safely between the fire engines and ambulances astride the runway.
Three hours later we took off again in a new plane with a new crew, but again, 45 minutes into the flight came more loud bangs. We went back again to Charlotte, and I made my second emergency landing in a single night. This time we stayed overnight in an airport hotel, and returned home uneventfully and safely the following night. The explanation given later was that unsecured cargo had moved in the hold, whose contents had been transferred from the first plane to the second, along with the passengers. Some of us had guessed as much.