That would be the system fan of the MacBook, by the way. 
The early releases of the MacBook apparently were having temperature problems, so much so that users reportedly experienced random shutdowns during the course of using their beloved notebook. It was found out that the fan was unable to change its speed as the temperature of the MacBook’s internals went up.
A utility was released by a third party developer that allows the user to explicitly control the fan’s speed as deemed necessary. Though the problem with the MacBook was fixed partly through a firmware update, the third party utility is still popular and is considered a must-have for MacBook owners.
What I’m referring to is the smcFanControl utility:
The program does just one thing: it lets you set several profiles where you can control the speed of the MacBook’s internal fan. It also provides a display of the current temperature and the fan speed:
If you’re paranoid and you want to make sure that the temperature of your notebook is maintained at the minimum, you can set the fan speed to maximum. However, one side effect of this is that the fan gets quite noisy at higher speeds.
Try it and feel the difference!