My 24" IMac freezes and makes a popping sound in the speakers. What could be wrong?

Good afternoon

I have an early 2008 24" imac, 2.8GHz Intel cor 2 duo, mountnain lion 10.8.2. From time to time it suddenly stops, and makes a popping noise from the speakers and then the computer is frozen, i have to press the power button. This is now happening more often, at first i thought it might be flash as it use to happen on youtube videos, but today it froze whilst usling skype.

I read on the forms, but my problem is not exactly as the ones on forums.

Does anyone have this problem? Could it be the harddrive failing. I did a clean install of mountain lion last night and it happened still, the hard drive is a 320gb Hitatchi

Thanks

I have included a youtube link of what it looks and sounds like.

http://youtu.be/jBo9g2Z8U0E