My six-year-old office computer (that I'm responsible for maintaining/troubleshooting) is making a buzzing noise when under load. By "load", I mean that it will buzz if I scroll a webpage quickly. The buzz is constant if the page has any Flash on it. And this isn't capacitor whine either -- people in the office have interrupted their conversations to ask, "What the hell is that sound?".
The PC is stable. The PSU is no-name (doesn't even list the wattage rating) and the voltage levels have always fluctuated slightly, but this is the first occurrence of buzzing.
Is this problem fixable, or am I justified in asking for a new PSU?
A PSU is not a consumer repairable device. Replace the PSU if that is where the buzzing is coming from. It could easily be electrical. Just make sure you back up any important data. A PSU failure can cause lots of issues elsewhere.
FYI, it is very dangerous to open a PSU. They store a tremendous amount of energy, enough to kill you outright or cause heart palpitations and kill you later on. I have seen this first hand when a tech opened up a CRT monitor and was accidentally grounded. Needless to say, the stored energy escaped through his body and stopped his heart.