hard drive or dvd drive, those are the first places i would check
what kind of power supply? i heard somewhere that the coolermaster(cant remember the brand) power supplies tend to be loud
Sounds like a fan somewhere in your system going to me. I would take off your case cover and try to look inside your case while it's running to find where the noise is coming from.
ive opened up psu's replaced fans in them only been shocked once