First of all, clean any dust out of the fan with compressed air since that could be causing the noise (and will cause your MBP to run hotter). Also clean the intake and exhaust vents if there is any dust.
The fan should have a small sticker in the middle, on the stationary part (not the hub that spins with the blades). If you peel back the sticker, you should see the bearing, and you can add some oil. Don't add too much or you will make a mess and possibly damage the fan. I have used a Radioshack precision oiler in the past, which is basically some oil in a needle-tip dispenser, but I have never seen adding oil fix a noisy fan. At best, it will make the noise quieter for a few weeks or months, but you will still most likely have to replace the fan if it bothers you. If the noise doesn't bother you, you don't have to replace the fan until it physically stops working, but I would replace it if it gets really loud since that can indicate that it is about to die.
