Simple .8-second fading beep. 500hz square wave. Took 2 seconds in audacity.
Couldn't find any good beeps for a countdown so I made one. 對數方波嗶嗶聲 聲道:1 類型:wav electronic、digital、computer、generated、beep、electric、noise 電子、數字、電腦、產生、嘟、電動、噪聲 資料來源:freesound ( https://freesound.org )