During the Power-On Self-Test (POST), the BIOS will check for problems. If a problem is found, the BIOS will activate an alarm, turn on LED indicators or display a message on the monitor.
For example these are know to result in audible alerts:
1 long and two short beeps - video configuration error
1 continuous long beep - overheat (Overheat LED will be on)
1 long beep and 1 short pause - memory not detected
There may be others as well. For example, a problem with the keyboard (like stuck key) may also result in an audible alert.
Best is to check the POST error on the monitor and/or check the system LEDs if you want to be sure what's causing the alert.