Frozen Display and Fire alarm Sound

Hello, I hope one of you can help me.

Since yesterday my iMac had this issue three times and I still don't know what the problem ist.

Out of nowhere (just listening to some music or using facebook) the display of my iMac freezes so I can't do anything, neither click on anything or use any key combination. Besides that a loud alarm occures that soundes like a fire alarm... it makes three loud noises, a break and again three loud noices and so on and so on.

(it's hard to explain so here's a link to a video I found on youtube: http://www.google.de/url?sa=t&rct=j&q=&esrc=s&source=web&cd=3&cad=rja&ved=0CE8Qt wIwAg&url=http%3A%2F%2Fwww.youtube.com%2Fwatch%3Fv%3DCmH1pHer66o&ei=CJELU_7HL8XR sgau9YCgAg&usg=AFQjCNE-XBrsHOSS7Ru03KKyjsTgzKEU0g&sig2=RuCY2xbs26tFQsHSvtWgpg&bv m=bv.61725948,d.Yms )

The only way to stop it is to turn off the iMac via the start button or to unplug the power supply.

Does anyone have an idea what the cause for this might be?

Thank you very much for your help!!!

Kind Regards

Birte

PS: Some further information on my iMac

21.5 Zoll, Mid 2010

3,06 GHz Intel Core i3

running OS X 10.9.1

no software updates available