Gopher, - as I recall from the early 747s, nothing to do with flaps - it came in about half way up the power curve and was roughly proportional from there on. Purely an engine noise and nothing to do with airframe config.
Maybe the enginers don't read this section of the board
I did ask a couple of my mates at the weekend, but none of them works on heavy commercials (rotorheads and air force singles only) so no joy there