Kaaaaaaw! originates from the VGA playthrough of God of War 3.
During this, every time Kratos falls from a high altitude or jumps great distances, the Wings of Icarus appears on his back and he glides through the air like a big majestic angry albino crow.
At one point during one of these glide flying, Ben or Fraser, saw the sillyness in this situation and accompanied it with a crow-like 'Kaaaaaaaw'. It was fondly accepted on the show and in the chat, and was after that used after almost every time Kratos flew in the game. This resembles the use of the fart sound when Kratos opens every single chest.
Zelda Skyward SwordEdit
This sound has been used in other games as well, and pretty much every time that Fraser encounters a bird of any kind. One of them are Legend of Zelda: Skyward Sword. As there is a lot of flying on a big loftwing, there should be no suprise that the 'Kaaaaaaaw' outburst was being used extensively.
During the brief Assassin's Creed episode, Fraser invoked the "Kaaaaaaw!" sound again when doing a leap of faith. This is because in game, a leap of faith is always accompanied by a soaring eagle and a bird call.
Arkham Asylum and Arkham City Edit
Fraser and Ben often employed the "Kaaaaaaw!" sound whenever Batman was gliding around.