iMac clicking noise

So yesterday when I got home from work I noticed that my iMac started to making some unusual noise. Its on the right side of the screen where the dvd drive is at. The noise occurs every 30 seconds then it stops.

It normally occurs when the computer is idle when nothing is on running: safari, itunes. Its the Mid 2011 iMac Base model.

Can someone please tell me what the problem with this computer please? Or is the clicking noise that loud normal.

thanks

 

Yeah I took it in today and there was so many people that they couldnt hear what was happening. So for the next 48 hours, I'll be iMac less lol. but yeah hopefully they are able to solve the problem.

On the hard drive, I barely have nothing in there other than my Microsoft Office installed and Skype. Other than that I just use the computer to surf the web and make videos from time to time which I delete.

Btw how much would Apple charged if I didn't have warranty to replace my 500gb hard drive?

----------

or it can be possible that it might be the fans next to the dvd drive?