I've seen a number of people complain about this after liquid spills. I think maybe a sensor stopped working? Hopefully somebody else can chime in with more information or maybe this will give you some more words to search on.
Umm. The fan on all MacBooks (minus the new 2015 one) will always be running. If it isn't, the MacBook is either off or the fan is dead. This is usually around 2,000 RPM or so depending on the model and it should be rather quiet. If it's running on high (5000+ RPM) it will get quite loud and it shouldn't be doing that unless it's running hot. Does it feel hot on the bottom or near the hinge? If so, something is making it run warm. Such as dust in the fan and heatsink.
If not, it could be something software related or a sensor as mentioned. You might try installing a second copy of OS X on another partition or external drive and booting off of that to see if it still does it. If it doesn't than it's software, if it does you've got something else going on.
Do something which Apple doesn't want you to do: open up the bottom plate and clear the dust out of the system with an air duster. There's probably a LOT of dust in the vent system that's preventing the Mac from cooling properly. I did it once, lost me 20 degrees Celsius off of the idle temps.
If you wanna know how to do that, Google it. There are plenty of articles/videos on how to open up a Mac and how to keep PCs in general dust free.