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.