I think (don't quote me on this
) but Microsoft have started to use a lower power and lower heat producing CPU so therefore the fans run slower creating less noise.
However most of the noise from the 360 comes from the DVD drive, so I guess they might have put a new one of those in too.
Were you playing a game off a disc when you noticed it was quieter? Or was it a demo downloaded from marketplace, as the DVD drive doesn't spin to play demos as they run off the HD so the noise is significantly less.