We need full system specs to have ANY chance of helping, anything hardware or software you installed lately, have you cracked the case to isolate the noise?
The software i installed lately were a haf 912, asus hd 7750, and a ocz 600w modstream x. From my old comp was a intel core 2 duo on a dell motherboard, with a dvd burner with a western digital caviar blue 300 gb 5000 speed/
Could be a few things. if it does not beep to start but then starts beeping after a few minutes its a thermal warning, open your computer, clean the dust & reapply heat transfer compound if needed.
If it beeps from the get to then it could be a CPU fan warning (normally you get a bios error message also tho)