Odd global bird-like sound, possibly related to Karthus

Yesterday, I was playing a game with some friends when we all started hearing a weird, bird like sound playing globally. It sounded almost like Valor. We asked, and the enemy team was hearing it as well. After about 20 minutes or so someone said they thought it was Karthus, and so I watched our teams Karthus and noticed that every time I heard the bird sound Karthus was using his dance. It is important to note that while every time the bird sound happened karthus was using his dance, it did not happen every time Karthus danced. Here is the game:

http://matchhistory.na.leagueoflegends.com/en/#match-details/NA1/2057800777/217750770?tab=overview