The reason why anxiety slowly builds up inside you when you hear sounds like a smoke detector or timer is because the beeps aren’t created by a natural transfer of energy. Humans evolved listening to sounds caused by vibrating surfaces that gradually dissipate, which helps the brain figure out what they are. The unchanging tone of a beep is suspicious to your brain, so it feels like speeding cars hitting a wall inside your head. Source
Source: http://didyouknowblog.com