[RicheemxX]

I'd skip buying a p/s tester just use a multimeter {or buy one instead}

but first off try jumping the p/s without the motherboard and other components follow this guide, no you don't need the switch just use a wire or paperclip.
http://www.gideontech.com/content/articles/196/1

I've never heard of a bad mobo causing your powersupply to buzz but I could somehow be causing an overload issue I guess?