Does your Macbook Air fan makes a repeating, predictable ticking noise? Does it get hotter than usual when watching YouTube video’s? Does iStat Pro reports a fan speed of 0?
A few taps with your finger help! If you have an older model (2009) tick on the right side. If you have a newer model (mine is late 2010) then tap on the right side. Don’t overdo it, but don’t be too gentle either.
See also this discussion: https://discussions.apple.com/thread/1860264