MacBook Ticking after prolonged fan usage

Ever since I bought my MacBook two years ago, I've noticed that there is a slight ticking noise that comes up intermittently after prolonged fan use. If I sit somewhere and the fans have to kick on overdrive for a long period of time (i.e.: using my MB on my bed rather than a hard surface) after the fans calm back down, I notice a ticking noise that's not consistent in its rhythm. It occurs about every 2-5 seconds and is faint, but enough to hear it in a quiet room. Reboot doesn't help, but shutting down and restarting does. Any clues?