I usually fly to and from South America (LIM) on American Airlines A300s. I always noticed that when the plane begins to move to the ramp (or just move), the engines make a spooling up sound (normal) and then make a sound as if they are spooling down (spooling up sound with a high pitch engine sound followed by a low pitched-hollow engine sound......But the thing is, the plane continues to move forward...even gain momentum!!). I sometimes fly on LAN Chiles 767s and noticed their engines don´t make that interesting sound. Hmmm.....Any thoughts anyone? I believe that American uses GE engines.