On Jan 14, 10:26 am, Alan wrote:
Several times for the past few years, while on a transatlantic flight
aboard a KLM 747-400, I begin to hear what sounds like a turbine
spinning up, almost like an engine start, but higher pitch...
Anyone know what it might be?

Thanks

Might be the air cycle machine. See this:

http://en.wikipedia.org/wiki/Air_cycle_machine

Dan