is that the stock PSU that came with the prebuilt?
because most of the time prebuilts come with lousy PSUs
what make and model power supply?
what is the 12v+ amp rating(s) of the PSU (will be on sticker on PSU)
the PSU is usually the first thing to look at
that buzzing in speaker could be coming from electro magnetic interference from a poor quality power supply
also PSUs can cause a mystery of problems and include destroying mobo and parts when they go bad