If canned air doesn't work, its your PSU fan, it needs re-oiling =)
Since you probably don't know what to do, and I really would not reccomend this unless you don't want to spend money and buy a new one; since the chance of electrocution by touching a capacitor is quite high.
Alright, this is what you do, I did this on my old generic PSU tonnes of times until I got my new computer (see sig ^_^) I don't think I'll need to be doing this at all. During this whole procedure exercise EXTREME caution!!!
Step 1. Remove the PSU covers
Step 2. Unscrew the PSU FAN(s) (whichever ones you think are making noise) [CAUTION DO NOT HIT TOUCH ANYTHING BUT THE FAN]
Step 3. Get some sewing machine oil, unpeel the sticker on the fan so you can see the little rotor thing, put 1-2 drops in there, no more, then put the sticker back
Step 4. Reattach the fan.
If done successfully, there should be no more whirr.
Like I said before, be extremely cautious.