Beeping Sounds (like an alarm)

My iMac keeps making an extremely persistent noise - a beeping sound exactly like an alarm clock. I have no idea why (I certainly didn't set any alarm settings). My Mac never gave me any trouble or problems - in fact, in the four years I've had it, this is pretty much the ONLY issue. And it's just a beeping sound.

For the record, it's not the loud buzzy beep either, that signifies a RAM issue. Its literally exactly the same as an alarm clock. So, what exactly is up?

Thanks!