the psu maybe undegoing too much stress when the system is booting as obviously everything is running at full speed. does it do it when you run stressful programs?

this next bit is potentially dangerous so i take no responsibility for any injuries you could get from doing this.

switch the psu off at the back and disconnect from the wall.
unplug the psu from the motherboard (i would unplug from other devices too but you can leave fans in)
test the psu by bridging pins 13 (black wire) and 14 (green wire) on the psu's motherboard connector. switch on at the wall and then the back of the psu. the psu will switch on.
obviously if it makes no noise, you know its stress on the psu causing it to do it. if it still makes a noise then chances are its knackered. you could test all your voltage rails while youre doing this as well.