About 10 years ago my wife and I were on a 747 from Los Angeles to Auckland late at night. It was the usual long roll prior to a long flight but as the aircraft rotated there was the most incredible scraping noise from the rear which lasted several seconds. The cabin went very quiet but we continued to climb and shortly levelled out. I whispered to my wife that I suspected a tail scrape and we'd probably dump fuel before returning. But no... a few minutes later we contined climbing and 12 hours later we landed in NZ! Nobody said a word so we still have no idea what it was other than possibly a rough surface at the end of one of LA's runways.