You don't say how old it is but after a while most power supply fans get noisy. All you have to do is pop open the power supply. ( YES uplug it first if your not adventurous ) I have taken many a fan be it power supply, ventilation fan, even a CPU fan or two out and lubed it and put i back. I have even had people I have done it for say it is quieter than when it was brand new.It's really simple besides I would open it up no matter what and see if it is just a wire that somehow is touching the blades slightly. IF that is the case just wire tie it out of the way the way it should have been done to begin with.When you open the PSU you have to take a couple of screws out that hold the fan to the PSU case side. SPin the fan and you 'll probably hear of feel its a little noisy.You'll see a little paper label over the fan bearing. Just gently pry it up and off and put a drop or two of 3 in 1 oil of lightweight oil on the bearing and spin the fan till it quiets down. It won't be loud like your hearing when it's running but you should notice a difference after oiling it. It doesn't take much use a drop or two. Once it gets worked into the bearing you replace the paper label ( it's really there as a dust cover also for the bearing) replace the fan in the PSU. pop the cover screws back in replace the PSU and you done.
Dont skimp on a PSU get a good one and when I say that I mean in the $100+ range. PC Power and Cooling are one of the best with quality and least with failure rates. We spend hundreds or more on the other parts so why cut corners on the most important (and over looked) one?