Originally Posted by PhilH
I think it happens when the flaps come down, but beware if you've never been on one before because it can be a bit scary (especially down the back). Its slightly surprising that they don't make an announcement.
Apparently it's the wind chime effect when the gaps between the bits of the flaps are just so during their travel. I'm also surprised by the lack of an announcement; I've flown quite a few QF 146s and they've always made a point of mentioning this.
I'm also slightly surprised that there aren't any acoustic fatigue issues from such a loud noise - maybe it's just not loud enough for long enough to damage anything.