You can unplug one fan at a time, then carefully power up your system each time and compare the results. If it is noticeably louder with one of the fans on, you've found the problem. In some cases, when lubrication won't do the trick, this troubleshooting will pay off later as you'll know which fan to replace.
Lubricating the fans
After you've decided which fan(s) to lube, the rest is pretty easy. Get your hands on some light lubricating oil, which you can find at any good electronics supply store. Make sure your computer has the power cord disconnected, so there's no chance of shock. Usually the fans will be easily accessible, though it is possible that you will have to remove some components to get access to a particular fan.
Computer fans have a lubrication point in the center of the blade assembly, which is usually covered up by a sticker. As demonstrated in
Now manually rotate the fan blades with your fingers, or a pencil may come in handy for this if the fan is very small. Turn the blades several rotations to get the oil to disperse evenly around the fan axel.
Wipe the outside of the fan where you peeled the sticker with a soft rag until it is dry and clean. Either replace the original sticker, if it will still stick, or find something else to cover the lubrication spot. Packing tape should work fine.
Repeat the procedure for all the fans you wish to lubricate. Now replace the case, plug your system back in and fire it up. If all went well, your PC should be noticeably less noisy.