I live 3km from SYD directly under the flight path. We regularly hear very similar sounds described above about 20-30 seconds after an aircraft has passed on final approach. It seems the smaller planes have a larger effect but particularly Virgin-Blue 738s where we hear a sound like the plane is flying overhead again, followed by weird whirling whistling sounds and sometimes a sound of a cracking whip.
Question is does these aircraft wake or vortices somehow descend as they seem to travel downwards and impact with the ground. Also I assume the bent up wing tip of the 738 is somehow related to increasing the affect. Larger 767s have a lesser effect and 747s and 380s none (although I will look out for these).
Thanks