Hearing an Alarm/Siren Sound

Sooo this might seem weird... but it has happened in 2 instances... both are when Im jungling. The first time I heard it was when I was playing Master Yi JG.... and everytime I alpha striked a jungle monster.... I heard an alarm/siren going off. I didnt know if my browser was opened so I alt-tabbed to look to see if a pop-up appeared but it didnt so I knew the sound from coming from League... the second time I heard it was when I was playing Vi and I Q'd a minion. Can anyone tell me what this is?