Why does my computer make a siren sound?

When needing to reinstall windows I decided to disable the hard drive in my bios. After turning it back on and having windows reinstalled it now makes a "siren" noise. How do I make it stop?
 
First, be a bit more specific: What exactly is making that noise? A system buzzer, or airflow turbulence?
 
Re:

I have no idea whats causing the noise! Someone told me I should remove the memory cadrs and then place them back in but that's no working..
 
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