Randomly during use, IMac freezes and then makes loud beeps

This has been happening for a while.

i Have a 21.5 inch, mid 2010 Mac. It was fine until around the start of the summer of 2015. My IMac will freeze, make loud scratching/crackling noises, then repeatedly beep 3 at a time. Ive searched other questions, but none of them have my problem.

ive never changed anything in this computer. Heck, I don't even know how to open it. I'd appreciate if someone could explain how to fix it.

thanks for reading.