Well, the noise from the engines depends on where you are sitting on the aircraft in relation to them. If you are in front of or next to the engines, you are more likely to hear the actual whine of the engines. If you are behind the engines and the wings, you will only hear the engines' exhaust, which makes the "windy" sound. I'm not sure where you were sitting on the aircraft, but it seems like you must have been in front of the engines on one flight, and behind them on the other.
I have a funny story with this one. My family and I would always get plane seats behind the wing on flights, and I always thought the engines sounded very quiet. On one American Boeing 757 flight out of
DEN, however, we got seats in front of the engines, and on takeoff, that well-known Rolls-Royce whine scared me half to death! I thought the engines were failing. But, at least now I know better about what noises to expect!
Anyway, hope this information helps!
Cheers,
flyusairways
[Edited 2006-08-07 07:08:38]