clock beep on my Imac

Hi! I've got a problem in my "vintage" 2007 Imac. It's a beep that sounds randomly while working. It's NOT A STARTUP BEEP. It's exactly a beep like the old digital Casio clocks from the 80's when they beep every hour. I've bben googling but I don't find anybody with the same problem... Maybe I'm the only one in the whole universe that got this problem??? I've called Apple but they couldn't tell me anything about it...

To show you exactly how that beep sound I have this video from Youtube that it's a kind of tutorial of how to get rid off that sound in the Casio clock.

So my beep it's exactly like this one at 1:32,

https://www.youtube.com/watch?v=Fl22dTiG5hA

The only difference is that I got only one, randomly.

So maybe somebody here could tell where that beep comes from...? Maybe it's from outer space??

Thank you all!