hmm, could be a number of things, is there anything metal that's touching the motherboard? any stray screws ?

nothing been put into the psu has it? a quick shake of your pc will let you know if there's any screws around as you will hear them (in theory unless they're jammed in)

check to make sure the voltage hasn't been changed on the back of your psu, although usually if that happens they go with a big flash and bang