Do you have internal mechanical hard disk drives? Try unplugging them and start the computer. It will not work correctly alright, just make sure if the buzzing is still there with this little test.
HDD's could get consumed the internal parts make a buzzing sound when they initiate. Some other others could have the sound from factory, but only really old ones as far as I know.
Also just in case, make sure you do not have an optical drive (e.g. CD or DVD) with a disk in it
One beep is normal. If it stops with the one beep when you boot up, you may need to check your bios settings. Does your pc's clock keep time/date properly?