I was on a flight from Denver to Boston on a UAL 757 and I also heard this noise. However, I heard it more than once. I heard it right before the aircraft took off and other times during the flight. I don't know if it is the same noise, but I have heard something similar.
"Bangkok Tower, United 890 Heavy. Bangkok Tower, United 890 Heavy.....Okay, fine, we'll just turn 190 and Visual Our Way