Modern versions of BIOS have developed diagnostic facilities, with the help of which you may identify defective or wrongly set hardware. BIOS can inform about such problems by sound signals, messages on a display or POST-codes.
An initial diagnostics of the computer is usually accompanied by one short audible tone which informs that the POST-procedure has completed successfully and the operating system is ready to boot. If a serious error has been found, the operation of a system will be stopped with audible tone and/or messages on a display. Audible tones are used in those cases when a fault does not allow the system displaying a message. They also may be used to alert the user to visual messages on a display.
It is noteworthy that some motherboards manufacturers may change the tone meaning or add the new ones. If you have heard a strange tone, try to search its decode in the board manual or contact a technical support service of your motherboard manufacturer.