Loud sparking noise from iMac

I have a problem with my iMac. I recently wiped my hardrive and re-installed OS X 10.8.2. Today I was listening to music in iTunes, when suddenly my mac starts making extremely loud crackling/sparking noises. My first response was pulling out the power kable, frightened that something may be burning. I checked the computer and there is no sign of anything burning. I know what a burning mac sounds and smells like, seeing as my previous iMac started burning from the inside during the middle of the night, and it wasn't even turned on. I never acctually found out why or how it happened. Apple were, for some reason, reluctant to tell me anything about it. After about 3 months (!!!) or so Apple were 'kind' enough to ship me a new iMac, the one I'm currently using.

I turned it on again and when I tried to play a song in iTunes it happened again, and I jumped up and pulled out the power kable.. again. This time I became quite scared. The noise is like something out of a horror movie, so loud. I turned it on again and now it's working fine, but I haven't tried opening iTunes again.

Anyone know what this could be?