How to fix loud fan noise?

The fan motors are sealed and unserviceable; the best you can hope for is a thorough cleaning of the blades and housing to eliminate dust deposits that may be unbalancing the assembly. Otherwise, the alternative is replacement; Apple regularly does that when they turn out noisy after awhile and are under warranty.

Parts & DIY procedures can be found at PowerBookMedic.com or IFixIt.com.

It would depend on whether you are referring to the normal sound of the fans spinning up to full speed, or if you have a broken fan that is making extra noise. My late 2008 MacBook Pro had the latter problem a few months ago and the out of warranty repair was about $100 as I recall from a local Mac repair center. That noise was a rattling.

Simple high speed air flow sound is normal if the machine heats up, and it's possible what are running is simply doing that, especially if the noise goes away shortly after you shut down the game and start doing less taxing work on the machine.

The good news, however, is that if your noise is a problem fan you should still be within the standard warranty period to get it swapped out. So it certainly makes sense to take it into Apple if you have a store nearby.

Take it back, as it is covered by the one-year base warranty. If you try to fix it, will void that warranty. Just be firm in insisting that the noise is unacceptable in a computer of this caliber and price range. Repair consists in replacing the fan (single one in the 13", two in 15" and 17"). May have to leave it overnight or more, so be sure to back up the hard drive and remove your pron stash for now... 😉

They will take it in and have a tech replace the noisy fan with a new one that will hopefully not devolve into a noisemaker like its predecessor. Any mechanical part is prone to failure over time. My now deceased Early 2008 15" MBP also developed quite the whine on the left fan about 5 months after I bought it; the local AASP replaced both fans for good measure and noise never came back during the 4 years afterward.

That is, provided the noise is indeed above and beyond normal wind noise for a propeller spinning at 6000RPM or more.

Yes, make an appointment at the Genius Bar. Or, if no Apple Store at your location, an Apple Authorized Service Provider.