PSUs are not expensive to replace and any computer shop ought to keep them in stock. But before you do that I would certainly check that it really is the PSU by detatching everything else and running it on its own.
I keep an ageing G3 iMac in the living room purely as an MP3 jukebox. It is probably the last completely convection-cooled, fanless computer there will be. Makes no noise at all except for whichever album it is playing at the time :-)
