the only thing i can think of is when you let the engine idle it starts to cool down the converter due to less exaust passing through(obviously lol) and then maybe the components inside and the heat sheild maybe contracting a bit, i don't get how this would make the sound on your video but thats the only explanation i could come up with, i did a search on google and wikipedia and other sites and read some stuff of catalytic converters (i'm kinda bored at the moment) but came up empty handed other than a thread on a cadillac forum of a guy describing the exact same thign as you and they seemed to think it was just the cooling off of the components as well but no one could say for sure so who knows.
edit: wstar beat me to it and said basically the same thing lol
