Why is my fan making so much noise?

I wouldn't be surprised if after these many years air intake would be difficult because of dust accumulated.

If it was never serviced before a good internal cleanup and thermal paste replacement might do wonders ;)

Edit by mayer

Remove the fan and clean out the vent, here's how:

MacBook Pro 13" Unibody Late 2011 Fan Replacement

Also, this model has a liquid filled heat sink that can develop a leak. Use the following guide to inspect it. Look for an oily type of reside on the logic board for confirmation that it has leaked. Check out the thermal paste when doing this.\MacBook Pro 13" Unibody Late 2011 Heat Sink Replacement